Cataphyll

Leaf bud of American Sweet gum (Liquidambar styraciflua); the cataphylls covering the bud show a little chlorophyll, but they will be shed instead of growing into photosynthetic leaves.

In plant morphology, a cataphyll (sometimes also called a cataphylla, or cataphyll leaf) is a leaf whose primary function is something other than photosynthesis. Cataphylls are at most trivially or transiently photosynthetic, and instead of photosynthesis the main functions of most types are storage, protection, or structural support. Many forms of cataphylls die in performing their function. Cataphylls such as bud scales often are shed after the need for them is past, but dead cataphylls that afford protection from weather or pests may be accumulated into long-lasting, thick coverings.

Forms of cataphylls

Some kinds of cataphylls perform a transient function, after which they die and may be shed. Those that are shed early are said to be caducous, but that term can apply to any organ that is shed early, not only leaves; for example, many Geraniums have caducous stamens. The sepals of Papaver species are shed during the very opening of the petals, and as such they are a dramatic example of caducous leaves. Many other forms of cataphylls, such as some spines, are persistent, but cannot perform their major function until they die, whether they physically get shed or not. Yet others perform indefinitely in the form of persistent structures that remain on the plant after they die. Examples of various kinds of cataphylls include bud-scales, bulb-scales, corm-scales, rhizome-scales, cotyledons, scaly bracts, spines and perhaps glochids. Each of these occurs in various forms and contexts; for example, bud-scales occur on various kinds of leaf or branch buds as well as on flower buds.

At all events, cataphylls are in general sacrificial organs; their function is not related to their own survival, but in direct or indirect support of the propagation of the parent organism.

The word cataphyll derives from the Greek; in context it means something like "leaf to be broken down", implying leaves that are discarded or consumed. In fact some forms of cataphylls, such as the leaves or leaf bases forming the tunic around a corm or a bulb, are retained after they have died, and the protective presence of their remains is their major function. Similar protective masses of dead leaves encircle the stems of some species of palm trees or aloes, but those are not usually regarded as cataphylls because their primary function while alive was photosynthesis, as is usual for leaves. Clearly, the precise limits of the definition of the concept is a matter of convenience in any given context.

Cotyledons as cataphylls

Acer pseudoplatanus seedling showing cotyledons that supplied the first photosynthetic function for the growing plant. They will soon drop off after the young leaves grow large enough to take over.

Welwitschia mirabilis, a young male plant. Its two leaves are the only leaves it ever will have, apart from its cotyledons, and in extreme contrast to cataphylls, those two living leaves must last its entire long life. However, one of its leaves already shows its first longitudinal split; Older leaves commonly develop enough splits to make the two leaves look like many.

Cotyledons are widely regarded as a class of cataphyll, though many kinds of cotyledon function as living tissue and remain alive till the end of their function at least, at which time they wither and may drop off. They begin as leaf rudiments and many kinds accumulate nutrient materials for storage, starting to give up their stored material as the plant begins to germinate. Some, such as the cotyledons of many legumes, conifers, and cucurbits, even develop chlorophyll and perform the first photosynthesis for the germinating plant. Logically it is stretching the term to call such an organ a cataphyll, because after all, most ordinary leaves also have limited life spans and drop off when exhausted, whether after one season or several. However, non-cotyledonous leaves are not normally regarded as cataphylls just because they do not live as long as the parent plant. If non-permanent leaves were termed cataphylls, then few leaves would be anything but cataphylls; the most convincing example of non-cataphylls then might be the two persistent leaves of a Welwitschia plant. Those two leaves of the Welwitschia must last the plant for its entire life span, typically many centuries, because the growth point, the apical meristem, dies early in the development of the seedling. Interestingly, apart from those two main leaves, the two cotyledons of Welwitschia also are persistent in a sense; their remnants form the basis of the obconical, sideways, growth of the mouth-like slit at the top of the stem.

Such special examples aside, most cotyledons are as it were, disposable once they have performed their transient function, with or without photosynthesis, and in suitable contexts may be regarded as cataphylls. It would not be practical to demand a sharply-distinct definition for a continuous range of widely-varied functions.

Spines as cataphylls

Opuntia compressa, showing a so-called spiny cactus. Each areole contains one or more fixed, large spines as well as a sheaf of glochidia. The spines are classic examples of cataphylls; the glochidia are debatable examples.

Opuntia basilaris showing dense tufts of glochids in their areoles. This is an example of a so-called spineless cactus; it has only detachable glochidia, and few fixed spines, if any.

It also is a matter of context and preference whether one regards any particular kind of spine as a cataphyll or not. The terminology for glochids in particular is confusing, as they are variously and arbitrarily referred to as spines, bristles and more. Morphologically only spines could strictly speaking be cataphylls, because the others are not leaves, but in the current context the point is hardly worth pursuing.

Some kinds of such defensive organs remain on the plant, whereas others, such as the glochids of Cacti in the sub-family Opuntioideae, not only detach when touched, but owe much of their very function to their tendency to remain stuck into the skin, respiratory system, or eyes of the victim. In either case, most of the tissue in most kinds of spines will be dead by the time the spine is ready for action, whereas at the start of its development a spine generally is soft and fleshy, alive, photosynthesising, and growing, but ineffective for defence. Spines and glochidia also are sacrificial organs in a sense.

Buds, flowers, and associated cataphylls

Dormant, imbricate buds on twig of "English Oak", Quercus robur. The buds are large and powerfully armored; in spring the buds of most of the oak trees in a district emerge abruptly at about the same time, creating a flush of foliage that can overwhelm the feeding capacities of early-spring herbivores long enough for most of the shoots to mature into resistant foliage full of defensive tannins and fibre

Searsia angustifolia (Rhus angustifolia) axillary and apical buds. Note that the perpetually active apical bud is naked, whereas the axillary buds, that may remain dormant, have imbricate cataphylls for protection.

Searsia angustifolia (Rhus angustifolia) axillary bud breaking through its cataphylls

Leaf support cataphylls and cataphyllous spathes on Monstera

Searsia angustifolia. Note the unsprouted axillary bud an the left, that may remain dormant indefinitely, protected by scaly cataphylls, whereas on the right in the axil of a still functional trifoliate leaf, there is a sprouted twig at least a season old.

Bud-scales and bract-scales (or scaly bracts) are leaves that have a specific protective function with at most trivial and transient photosynthetic function; they are vital protection against pests and climate, especially during periods of dormancy. Their most spectacularly specialised examples are the often precisely imbricate bud scales of the broad-leaved trees of boreal forests.

In warm temperate climates one also finds some plants with armored buds, even if there is no winter dormancy. For example, many species of sumach indigenous to temperate zones, such as (Searsia or Rhus), have naked apical buds that continue growing throughout the active season and never go into a state of dormancy before their stems lapse into senescence. However, each leaf on a shoot has an axillary bud, and not every axillary bud begins to grow as soon as it is formed; instead it might take years before there is occasion for such an axillary bud to open; it might never open at all. For protection during its dormancy, if any, the first few leaves of each axillary bud grow into a snug imbricate covering of cataphyllic bud-scales soon after the bud forms. In this article, pictures of Searsia angustifolia illustrate the effect.

There are yet other classes of bud protection; large leaves of tropical plants without any dormant stage to speak of, such as Philodendrons, often develop within an unusually large protective cataphyll, possibly functioning largely as scaffolding for the growing leaf, or protection from wind during the period when their tissue is tender and their fibres are undeveloped. The protective cataphylls curl back and dry out as the leaf opens and matures, after which they often are shed.

Calyx of unripe Cape gooseberry, Physalis peruviana. The sepals enlarged rapidly after pollination and form a covering to protect the fruit in its unripe state.

The protective bracts growing in and around the inflorescences of Musa species, such as bananas, amount to cataphylls protecting their flowers and young fruit. They die and may be shed as the inflorescence matures.

Calyx of ripe Cape gooseberry, sepals on the ripe fruit turn papery and fragile after their protective function lapses.

One also could argue for regarding the sepals of some plants as cataphylls. Consider two contrasting examples — the sepals of the "Cape gooseberry" unite to form a persistent protective shroud around the fruit, becoming papery and brittle as the fruit ripens, though the cover is not actually shed. Conversely the sepals of most poppies are strictly caducous; after having protected the flower bud, they are physically shed during the process of the opening of the petals.

Bulb-scales

Onions with cross-section, showing roots, scales, and tunic layers (onionskin).

Ixia corms, one sprouting and showing tunic layers, the other with tunics removed, inverted to show the root base. The stem is solid, not made up of leaves like those making up the onion bulb.

Bulb scales, such as those comprising the bulb of an onion or Amaryllis, are cataphylls primarily in that that part of the organ typically never acts as photosynthesising leaf tissue. However, that is something like special pleading, because many or most of the bulb scales were at one time just the non-photosynthesising leaf bases of photosynthesising, apparently deciduous leaves. It is in fact possible for most bulb scales to develop photosynthetic tissue, and some plants adapted to arid, sun-baked conditions on stony soils, such as some Karooid species of Ornithogalum, commonly lie on the soil surface, with the exposed bulbs as significant photosynthetic organs. However, most bulbs are subterranean, with scales that never see the light. In such species only the visible parts of the leaves are green and deciduous, and the bulb scales stay in place as living, non-photosynthetic storage organs. On most bulbs some scales are the bases of leaves that never were photosynthetic, but immediately formed cataphylls that function either as storage organs or as protective scales, or both in turn. New leaves in true bulbs, botanically speaking, are produced from the centre, and as they grow, they force the older bulb scales outwards. The outermost scales of the plant yield up their stores to the plant each season and as their depleted tissue dies, the residue gets added to the bulb's protective tunic. Only then does the dying leaf complete its role as a cataphyll in the usual sense of dying and being discarded as it breaks down. Most onions live for only a few years, but for example undamaged Amaryllis or Boophone bulbs in the wild can grow indefinitely, and each new bulb scale can take many years to reach the outside of the bulb and merge into the tunic.

Corm-scales

Crocosmia corm with tunic stripped partly off to show its constitution of the basal parts of leaves arising from nodes on the corm. Such leaves, especially early leaves that never performed much photosynthesis, amount to true cataphylls

Corm of Crocosmia, split to show tunic and leaves, as well as a new corm growing from a bud on the cortex of the old corm

Like bulb-scales, corm-scales are largely the basal parts of the photosynthetic leaves that show above ground. Some species of cormous plants, such as some Lapeirousias also produce cataphyllous leaves that act as practically nothing more than tunic leaves for the corm. Unlike bulb-scales however, the corm tunic has no significant storage function; that task is left to the parenchyma of the cortex of the corm.