DEFINITION AND CLASSIFICATION OF LEAVES. 597 



close above the cotyledons. But later on buds are also developed on this primary- 

 shoot, and most usually close above the place where leaves arise from the axis of 

 this shoot. Many of the buds elongate and themselves become shoots, and we then 

 say the shoot has formed branches. Some of the buds, however, only undergo a 

 slight extension, and we distinguish between long branches and short branches, to 

 which we shall return subsequently. 



Of special interest to us here are the leaves of these shoots, the whole of 

 which are comprehended under the general term shoot-leaves. They exhibit 

 much greater diversity in form than do cotyledons, and this is quite intelligible, 

 since the tasks required from a shoot are much more numerous, and the allot- 

 ment of various functions to the leaves inserted on the shoot at different heights 

 necessitates a greater wealth of form. But the extraordinary abundance of 

 shapes makes it necessary to group the shoot-leaves according to their origin, 

 their mutual position, and their succession in time. We have long ago complied 

 with this requirement, since we distinguish them as scale-leaves, foliage-leaves, 

 and floral-leaves. Lowest on the shoot we see the scale - leaves. They are 

 developed earliest, and their rudiments are frequently to be seen even in the 

 bud from which the shoot is produced. They generally appear only as the 

 sheathing portions of leaves — as scales devoid of chlorophyll, and exhibit relatively 

 small dimensions. Following these scale-leaves further up the shoot are the 

 foliage-leaves; these arise later, are larger in size, and generally developed with 

 green laminae directed towards the sun's rays as foliage. Finally, above these, 

 the floral-leaves, which form the termination of the series of leaves growing on 

 a shoot, and take part either directly or indirectly in the production and union 

 of the sexual cells. One and the same shoot does not always bear the three 

 kinds of leaf -structures one above the other at the same time. There are some 

 plants whose shoots never bear foliage-leaves, and it is a very common occur- 

 rence for a compound plant -body to develop no floral-leaves on one shoot, and 

 no foliage -leaves on another, while in the Lathrophytum Peckoltii, one of the 

 Balanophorese described on p. 196, only floral-leaves are formed, and neither a 

 foliage-leaf nor a scale-leaf has ever been seen on this plant. 



The leaves, hitherto distinguished only with regard to their succession in age, 

 relative position, and insertion on the stem, must now also be described in 

 connection with the shapes which they assume and the functions which they 

 fulfil. Touching this I cherish the conviction that the special form is always 

 correlated with a special function, and that the recognition of the relation of 

 shape to the performance of work is the highest problem of the science of plants. 



