DEFINITION AND CLASSIFICATION OF LEAVES. 595 



the drones, and the queen are of different forms in accordance with their different 

 tasks, as demanded by the division of labour — so the leaves, which agree in their 

 first stages of development, exhibit, in their fully formed condition, another con- 

 struction in accordance with the function assigned to them. Hence we come to 

 this conclusion: — the variety of the tasks accomplished for the prosperity and 

 maintenance of the whole plant, and the consequent division of labour, necessitate 

 the metamorphosis of the leaves in each plant-body. 



From what has been said it follows that a definition of the botanical leaf must 

 be connected with the first stages of development. At the earliest stage each leaf 

 appears as a lateral swelling or protuberance below the growing point of the stem. 

 It arises from the peripheral portions of this region, which are still in a state of 

 active growth. The growth of the leaf is limited, so that in respect of their 



j development, we may say that leaves are laterally developed members of limited 



I growth, which spring in geometrical succession from, the outer layers of tissue 



I helow the growing point of the stem. 



In many foliage-leaves we can plainly distinguish a plate-like, outspread, green 

 portion, traversed by lighter veins, the blade {lamina), also a firm and stalk-like 

 support for this blade, the leaf-stalk (petiolus), and, finally, that portion which 

 connects the leaf-stalk with the part of the stem in question. In many plants this 

 latter portion is widened, grooved, and occasionally provided with a membraneous 

 border, so that the stem is then surrounded by this portion as the blade of a dagger 

 is by the sheath. This part of the leaf has in fact been termed the sheath (vagina). 

 Where the leaf projects from the stem there are frequently two outgrowths, one on 

 the right, the other on the left of this sheathing portion. These have generally the 



1 form of membraneous scales (see fig. 92^). They are often dilated, as, for example, 

 m the Tulip-tree (fig, 91), and usually fall oflf when the leaf at whose base they are 

 inserted is fully developed. In other plants they have the form of small lobes or 

 auricles, are coloured green, and are retained as long as the leaf remains connected 

 with the stem. These structures are called stipules (stipulce). 



Leaves in which the blade, leaf-stalk, sheath, and stipules are plainly developed, 

 lare on the whole less frequently met with than those from which one or other of 

 these portions is absent. Often no trace is to be seen of the stipules. Sometimes 

 only the leaf -sheath is present in the form of a concave, husk-like scale. In other 

 instances the leaf-stalk is absent, and the blade is then situated directly on the 

 stem (fig. 14); or it may happen that the green tissue of the blade surrounds the 

 whole stem like a collar, so that it might be thought that the stem had been stuck 

 through, or had grown through this leaf. If two or more of these leaves with 

 sessile blades arise together, they may be fused into a bowl or cup, being partially 

 or wholly united, and then again it looks as if the stem from which these leaves 

 arise has been thrust through the middle of the united leaf-group (see fig. 56). 

 Occasionally the green tissue of sessile leaf -blades is seen to be continued down the 

 stem in the shape of two green bands or wings. For the forms here only very 

 briefly described, the botanical terms are sessile leaves (folia sessilia), perfoliate 



