This section is from the book "The Gardener V2", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
Leaves, when perfect and fully developed in flowering plants, consist of two parts : the lamina (limb), or disk - and the petiole, or foot-stalk; the latter in many cases being articulated or jointed with the branch or stem, so as to be readily detached without laceration when the leaf begins to decay. Leaves originate around the growing apex of the stem - they are never terminal organs - and are expansions of the bark immediately below the origin of regular leaf-buds, and appendages of the axis. They are sometimes opposite, as in labiate plants, such as Coleus and Wood Betony; alternate, as in Ivy and the Common Garden Pea; verticillate, as in the common Bedstraw (Galium), etc. As I mentioned before, petioles are foot-stalks, and the petiole is the channel through which the vessels of the leaf are connected with those of the stem : it is formed of one or more bundle of spiral vessels and woody tissue, enclosed in a cellular integument. It is often absent, and then a leaf is called sessile, as the well-known spotted Orchis. The leaves of the Common Primrose (Primula vulgaris) are also sessile.
When the petiole becomes dilated and hollowed out at its upper end, the lamina being articulated with, and closing up its orifice, as in Sarracenia and Nepenthes, it is called a pitcher or Ascidium; if it is unclosed, and is a mere sac, as in the Utricularia, it is termed ampulla. Sometimes the petiole has no lamina, or is lengthened beyond the lamina and retains its usual cylindrical or taper figure, but becomes long, and twists spirally; such a petiole is called a tendril. Excellent examples of tendrils are furnished in the Vine, Pea, and Clematis. The form of the simple leaf is extremely variable. The leaves of the tall Convolvulus are cordate; of the Ground Ivy (Glechoma), reniform; of the Mezeraeon and Plantain, lanceolate; of the Daisy, spathulate; of the Iris, ensiform; of the Sheep's Sorrel, sagittate; of the Arum, hastate; of the Whortleberry, retuse; of the Tulip-tree, truncate; of the Horse-chestnut, digitate; of the Passion-flower, palmate; of the Christmas Rose, pedate; of the Water Milfoil, pectinate.
In reference to size, the leaf varies from a point to an expansion of immense magnitude. The leaves of the Palmyra Palm (Borassus flabelliformis) will each shelter twelve persons; beneath a leaf of the Corypha umbraculifera, a person on horseback can be completely sheltered; and a leaf of the Pandanus longifolia has often been grown 15 feet long in this country. Again, on the contrary, leaves are almost as thin as a hair : native examples of this are furnished by some of our aquatic Ranunculi. The leaves of Myriophyllums are also extremely small. By the foregoing remarks it will thus be seen that in consistency leaves vary from a gossamer tissue to the enormous ones of the Palms, etc, I have mentioned. A perfect plant consists almost entirely of leaf-formations, which are as follows: (a) The lower stem-leaf; (b) true stem-leaf; (c) upper stem-leaf; (d) calyx-leaf; (e) flower-leaf; (f) stamen-leaf, and (g) fruit-leaf formations. An explanation of the above seven terms is necessary. The first formation (a) is characterised in form by a broad base and limited height; in substance by a frequently fleshy cartilaginous or leathery consistence, and a dark colour.
These characters show that this, the lower stem-leaf formation, is wholly or partially excluded from the influence of light and air, and serves for the fastening of the plant in most cases. (b) The second formation, or that of the true stem-leaves, is generally a very extensive structure, characterised by the multiplicity of its organs, which are properly called leaves, and are distinguished from the lower stem-leaves by a greater longitudinal extension, with less breadth of base, expansion at the upper and contraction at the lower ends, a more membranous nature, and a green colour. The third (c), or upper stem-leaf formation, which consists principally of sheaths, bracts, glumes, etc, in some degree similar to the first formation, but are distinguishable from that formation by the much more delicate structure and narrower base; they present but little that is strikingly remarkable, in consequence of their very small size. The fourth (d) formation is that of the calyx-leaf, or more popularly known as sepals, and are much more massive, coarser, greener than the last formation (c); they have a broader base, are equally destitute of any incision, and rarely possess an expansion.
The fifth (e), or the formation of the flower-leaf or petals, is especially distinguished from all others by the delicacy of its tissues, as well as by the purity and diversity of colours. Flower-leaves, or petals, are generally longer than calyx-leaves, but narrower at the base; in some cases the corolla resembles a calyx, and vice versa. The sixth (/) formation consists of the stamen-leaves, which are the smallest and most remarkable leaves of the flower, with a decisive development of petiole and small expansion, which passes into the bag-like enlargements of the lateral parts or anthers. The last or seventh (g) formation is that of the fruit-leaf or carpels, which form the uppermost storey of plant-structure. Here (as in the last-named formation) the leaf-like character is less obvious, principally because the individual leaves of this formation are always crowded together more than those of any other, and generally even grown together, forming a cavity in which the axis terminates in numerous radiations; they are thicker, larger, and greener than the other leaves; they rise from a smaller base, but expand immediately, while the upper part contracts in a petiolar manner, forming the style.
These leaves have a longer duration than any others, and still continue to be developed when the others are for the most part dead. In these leaves, together with their enclosures, that in the first instance form what are termed fruit - buds, from which afterwards the fruit is developed, various modifications of leaves are often seen; for instance, the "prickles" of the common Dog-rose are merely modified leaves. William Roberts.
(To be continued).