596 DEFINITION AND CLASSIFICATION OF LEAVES. 



leaves {folia perfoliata), connate leaves (folia connata), and decurrent leaves (folia 

 decuiTentia), of which terminology this explanation must be given, that in earlier, 

 and indeed even in modern times, the leaf-blade — as the most noticeable part of the 

 leaf — has been in describing plants, shortly called " the leaf " (folium). 



The classification of leaves with regard to their point of origin from the stem is 

 of particular importance, and in this connection we must first of all distinguish 

 between seed-leaves and shoot-leaves. The former only occurs in the embryo, the 

 latter in all those structures comprised under the term " shoot ". The embryo 

 which has developed from the fertihzed egg-cell in the embryo-sac— in a manner 

 which has yet to be described in detail — presents in many instances a tissue-body 

 in which as yet no trace can be recognized of a differentiation into stem and leaf, 

 or rathei-, the embryo, when it leaves the fruit-capsule, is like a stem in which 

 all indication of leaves is absent, e.g. in several thousand orchids, the numerous 

 Balanophorese and Eafiiesiaceae, species of Broom-rape (Orobanche), Winter-green 

 (Pirola), Bladderwort (Utricularia), Bird's-nest (^lonotrojM), Dodder (Cuscida), 

 and many other epiphytes, saprophytes, insectivorous plants and parasites, as well 

 as many plants living together symbiotically. In the majority of instances 

 however, a distinct differentiation can be recognized in the embryo hidden in the 

 seed, and one or two leaves may be seen issuing from the tissue-mass which forms 

 the axis of the embryo. These are the seed-leaves or cotyledons. The short axis or 

 stem-portion from which the seed-leaves originate, and which looks like the 

 pedestal of the cotyledons, is called the hypocotyl. At one end of the hypocotyl a 

 tissue-mass is developed, termed the radicle (radicula); at the opposite end a tissue- 

 mass named the plumule (plumula). (See figs. 141 ^ and 141 ^.) The plumule is 

 situated above the place where the cotyledon, or pair of cotyledons, issue from the 

 hypocotyl. It is the rudiment of a new portion of the stem, which is situated above 

 the cotyledons, and is called the epicotyl. The epicotyl thus originates from the 

 apex of the hypocotyl, and the boundary between these two portions of the stem 

 is the place of origin of the cotyledon, or pair of cotyledons. 



The epicotyl in the resting seed is frequently only a tiny knob or cone, on 

 which no indications of leaves are yet to be seen. In the majority of instances, 

 however, distinct, although as yet very small, leaflets may be found on it, and 

 where this is not the case swellings sooner or later arise which are the leaf- 

 rudiments. Each short stem-structure, with closely-crowded and overlapping leaves 

 or leaf -rudiments, is called a bud (gemma); consequently the plumule is a bud, in 

 fact it is the bud of the embryo, which arises from the apex of the hypocotyl. 

 This bud, at the germination of the seed, elongates; its axis, hitherto very short, 

 stretches; the overlapping leaflets are separated, new leaves arise under the 

 growing-point, and so the bud develops into a structure termed a " shoot " 

 (innovatio). The bud is accordingly the primary groundwork of a shoot, and when 

 considei-ing the form of a compound plant-body, special regard should always be 

 paid to the places where the buds originate. The first bud, which is established in 

 every plant-body at the commencement, is situated at the apex of the hypocotyl, 



