5.3: The Leaf
The first and ultimate goal of every plant is photosythesis. If a plant is multicellular, it usually develops relatively large, flat structures which goal is to catch sun rays. Terrestrial plants are no exception; most probably, they started to build their body with organs similar to present day leaves.
A leaf is lateral photosynthetic organ of shoot with restricted growth . Its functions are photosynthesis, respiration, transpiration, and synthesis
of secondary chemicals. Features of a leaf (i.e., characters help to distinguish it) include having a bud in the axil, not growing by apex, not producing new leaves or shoots, and having hierarchal morphology (see below).
Morphology of the Leaf
Morphology means external, well visible structural features whereas anatomy needs tools like a scalpel and/or microscope to study needs tools like a microscope and/or scalpel. Leaves are very important in plant morphology. The ability to describe the leaf is a must even for novices in botany.
In all, plants are fractal organisms , like Sierpinski triange (Figure \(\PageIndex{3}\)). All fractals are self-similar (Figure \(\PageIndex{4}\)), and plants are no exception. Self-similarity, or “Russian doll effect” means that almost every part of plant may be a part of the bigger complex, this bigger one—the part of even bigger system, and so on. This is what we see in leaves as levels of hierarchy . * Simple leaves simple leaf leaf with one level of hierarchy have just one level of hierarchy whereas compound leaves have two or more levels of hierarchy. * Compound leaves compound leaves leaves with two or more level of hierarchy are sometimes mixed with branches but there are many other characteristics which allow to distinguish them (Figure \(\PageIndex{2}\)).
To describe leaves, one should always note the level of hierarchy like “on the first level of hierarchy, the shape is ..., on the second level of hierarchy, the shape is ...” As it was mentioned above, leaf hierarchy is similar to Russian dolls: every smaller doll has a bigger doll (next hierarchy level) outside. For example, if the leaf is compound (consists of multiple leaflets), the overall shape of it could be, saying, round (circular) but the shape of individual leaflet of the very same leaf could be ovate (Figure \(\PageIndex{5}\)). As a result, the description will say that on first level of hierarchy the leaf is ovate, and on the third level—circular.
There are three types of leaf characters: general, terminal, and repetitive. General characters are applicable only to the leaf as a whole are only applicable to the whole leaf. Terminal characters are applicable only to the leaf terminals (leaflets) are only applicable to the terminal leaflets. Terminals are the end parts of leaves, they do not split in smaller terminals; clover leaf, for example, has 3 terminals. Lastly, repetitive characters repeat on each level of leaf hierarchy. General and terminal characters do not depend on hierarchy. Repetitive characters are applicable to the leaf parts on each level of hierarchy may be different on each step of hierarchy.
General characters of leaf include stipules and other structures located near leaf base (Figure \(\PageIndex{6}\)): sheath (typical for grasses and other liliids) and ocrea (typical for buckwheat family, Polygonaceae).
Repetitive characters are the shape of the leaf (Figure \(\PageIndex{7}\)), leaf dissection, and whether the blade is stalked (has petiole) or not.
Terminal characters are applicable only to terminal leaflets of leaves. These characters (Figure \(\PageIndex{9}\)) are the shape of the leaf blade base, the leaf tip, the type of margin, the surface, and the venation. The base of the leaf blade could be rounded , truncate (straight), cuneate , and cordate . The leaf apex could be rounded , mucronate , acute , obtuse , and acuminate . Leaf margin variants are entire (smooth) and toothed : dentate , serrate , double serrate and crenate .
Leaf veins are vascular bundles coming to the leaf from stem. Frequently, there is a main vein and lateral veins (veins of second order). There are multiple classifications of leaf venation; and example is shown on the Figure \(\PageIndex{10}\).
Note that in dichotomous venation, each vein divides into two similar parts which is known as dichotomous branching. The example of dichotomous venation is the leaf of maidenhair tree, ginkgo ( Ginkgo biloba ). Another frequently segregated type of venation is parallellodromous , but in essence, this is acrodromous venation in linear leaves (for example, leaves of grasses) where most of veins are almost parallel.
To characterize the whole leaf, one might use the following plan:
1. General characters (leaf as a whole):
(a) stipules (present / absent, deciduous / not, how many, size, shape);
(b) base (sheath / no sheath, ocrea / no ocrea)
2. First level of hierarchy: repetitive characters:
(a) symmetry (symmetrical / asymmetrical);
(b)shape;
(c) dissection;
(d) petiole (presence and length)
3. Second level of hierarchy
4. Third level of hierarchy, and so on
5. Terminal characters (leaflets):
(a) base of leaf blade (rounded, truncate, cuneate, cordate);
(b) apex (rounded, mucronate, acute, obtuse, acuminate);
(c) margin (whole, dentate, serrate, double serrate, crenate );
(d) surfaces (color, hairs etc.);
(e) venation (apo-, hypho-, acro-, ptero-, actinodromous)
Heterophylly h refers to a plant having more than one kind of leaf. A plant can have both juvenile leaves and adult leaves, water leaves and air leaves, or sun leaves and shade leaves. A leaf mosaics refers to the distribution of leaves in a single plane perpendicular to light rays, this provides the least amount of shading for each leaf.
Leaves have seasonal lives; they arise from the SAM through leaf primordia , and grow via marginal meristems. The old leaves separate from the plant with an abscission zone .
The famous poet and writer Johann Wolfgang Goethe is also considered a founder of plant morphology. He is invented an idea of a “primordial plant” which he called “Urpflanze” where all organs were modifications of several primordial ones. In accordance to Goethe’s ideas, plant morphology considers that many visible plant parts are just modifications of basic plant organs.
Modifications of the leaf include spines or scales for defense, tendrils organ modifications using for climbing for support, traps , “sticky tapes”, or urns for interactions (in that case, catching insects), plantlets for expansion, and succulent leaves for storage. Plantlets are little mini plants that grow on the main plant and then fall off and grow into new plants; the most known example is Kalanchoë (“mother of thousands”) which frequently uses plantlets to reproduce. Plants that have insect traps of various kinds are called carnivorous plants (in fact, they are still photoautotrophs and use insect bodied only as fertilizer). Several types of these are the cobra lily ( Darlingtonia ), various pitcher plants ( Nepenthes , Cephalotus , Sarracenia ), the butterwort ( Utricularia ), the sundew ( Drosera ), and the best known, the Venus flytrap ( Dionaea ).
Anatomy of the Leaf
Anatomically, leaves consist of epidermis with stomata, mesophyll (kind of parenchyma) and vascular bundles, or veins (Figure \(\PageIndex{12}\)). The mesophyll, in turn, has palisade and spongy variants. Palisade mesophyll is located in the upper layer and serves to decrease the intensity of sunlight for the spongy mesophyll, and also catches slanted sun rays. The palisade mesophyll consists of long, thin, tightly arranged cells with chloroplasts mostly along the sides. The spongy mesophyll are roughly packed, they are rounded and have multiple chloroplasts (Figure \(\PageIndex{11}\)).
When a typical stem vascular bundle (which has xylem under phloem) enters the leaf, xylem usually faces upwards, whereas phloem faces downwards. Bundles of C\(_4\)-plants have additional bundle sheath cells in their vascular bundles.
The epidermis includes typical epidermal cells, stomata surrounded with guard cells (also optionally with subsidiary cells), and trichomes. Almost all epidermal cells are covered with waterproof cuticle, rich of lignin and waxes.
The stomata assists in gas exchange, cooling and water transpiration. There are two guard cells paired together on each side of the stoma. These guard cells are kidney beans shaped and have a thicker cell wall in the middle. The thicker cell wall on the inside makes use of the so-called “bacon effect” (when bacon slice curved on the
frying pan) because thinner part of the cell wall is more flexible and therefore bends easier. The same curving effect might be seen in blowing air balloon with the piece of scotch on one side. The opening of the stoma starts from K\(^+\) accumulation, then osmosis inflates guard cells, and finally the uneven cell wall facilitates the opening of stoma. The stoma closes when the potassium ions exit the cell and water amount decreases in its vacuoles (Fig [STOMA]).
In most cases, the lower epidermis contains more stomata than the upper epidermis because the bottom of the leaf is cooler and transpiration there is safer. A similar logic is applicable to trichomes (hairs): they are also more frequent on the lower side of the leaf.
Ecological Forms of Plants
When plants adapt to the particular environment conditions, leaves usually respond first. Conversely, one can estimate the ecology of plant simply looking on its leaves.
In regards to water, there are four main types of plants: xerophytes, mesophytes, hygrophytes, and hydrophytes. Xerophytes are adapted to the scarce water (Figure \(\PageIndex{12}\)), they could be sclerophytes (usually with prickly and/or rich of sclerenchyma leaves) and succulents (with water-accumulating stems or leaves). Mesophytes are typical plants which adapt to regular water. Hygrophytes live in constantly wet environment, their leaves adapted to high transpiration and sometimes even to guttation (excretion of water drops). Hydrophytes grow in water, their leaves are frequently highly dissected to access more gases dissolved in water, and their leaf petioles and stems have air canals to supply underwater organs with gases.
In regards to light, plants could be sciophytes or heliophytes. Sciophytes prefer the shade to sunlight, their leaves contain mostly spongy mesophyll. Heliophytes prefer the full sun and therefore have leaves filled with palisade mesophyll. The intermediate group are “partial shade” plants.
Halophytes, nitrate halophytes, oxylophytes, and calciphytes are ecological groups adapted to the over-presence of particular chemicals. Halophyte plants are frequent, they accumulate (and look similarly to succulents), excrete or avoid (which looks like sclerophyte) sodium chloride (NaCl). They grow in salty places: sea shores, salt deserts and solonets prairies. Nitrate halophyte plants grow on soils rich in NaNO\(_3\). Oxylophytes grow in acidic soils, whereas calciphytes plants adapted to over-presence of CaCO\(_3\) grow in basic, chalk soils rich in CaCO\(_3\).
Leaves will also reflect adaptations to the substrate, ecological forms named psammophytes (grow on sand), petrophytes (grow on rocks), and rheophytes (grow in fast springs). The latter plants frequently have serious simplifications in their body plan, their leaves and stems are often reduced to form a thallus-like body.
Parasitic plants could be classified in mycoparasites, hemiparasites, and phytoparasites. Mycoparasitic plants feed on soil fungi, phytoparasitic plants are either plant root parasites or plant stem parasites lacking chlorophyll and photosynthesis. Hemiparasitic plants are those which still have chloroplasts but take the significant part of water and even organic compounds from the host plant (like mistletoe, Viscum ).