THE AXGIOSPERMAE : LEAVES 947 



Are we then to abandon all categories and treat every plant structure as 

 an independent entity ? Not entirely so. A survey of the types of lower 

 plants, such as we have made in former chapters of this work, shows us that 

 the categories of stem, root and leaf in the Higher Plants are not primitively 

 distinct but are differentiated from one primary structure, the axis, which, 

 as we have previously suggested, may be derived from the primordial cell- 

 filament. They have been differentiated moreover in very many ways and 

 in very various degrees, so that fine-drawn distinctions between them must 

 be artificial. Nevertheless, if we compare many types of Higher Plants we 

 find certain regularities in the relationship of organs, associated with the 

 greatest diversity of individual form, which justifies us in using the old 

 categories as general descriptive terms, although without their old rigidity, 

 and thus simplifying and clarifying our ideas, while leaving room for the 

 recognition of cases which cannot be fitted into the framework of anv formal 

 definition. 



We have already applied this principle of relationship when we described 

 the stem as the organ which bears the leaves. What then are we to include 

 under the term " leaf " ? We may agree to apply the term to those structures 

 which arise as lateral outgrowths from the superficial tissues of the stem 

 without any subtending organ, and are arranged in a definite geometrical 

 order. Further, we may add as normal, though not invariable characters, 

 that leaves are generally of bilateral symmetry and of limited growth, that 

 they subtend buds in their axils and that they do not directly bear other 

 leaves though they may produce buds. Thus, if we seek to apply the name 

 to a particular organ we must be guided by the generality of its characters, 

 recognizing that Nature has no regard for our subjective limitations, and that 

 a given organ may contravene one or even several of the above characters and 

 yet be most appropriately classified as a leaf. 



^luch was made at one time of the restriction that a leaf could never be 

 terminal on the axis. There are a number of instances, both normal and 

 abnormal, in which leaves do assume this position by displacing or replacing 

 the true stem apex. Apart from such disputable structures as carpels or 

 stamens, which are frequently terminal, there are terminal leaves in 

 Polygonatum, which owe their position to the abortion of the apical bud. 

 Among the Gymnosperms, Pimis monophyUa also appears to have its single 

 leaves terminal on the spurs. Yet in this case also, traces of an abortive apex 

 may be found, so that the terminal position is only secondary. In Juncus, 

 " terminal " leaves may cover and include the stem apex in their bases, but 

 they do not replace it. Instances which have also been cited in Curydalis 

 and in a Bamboo appear to be similar to the foregoing, i.e., the apparently 

 terminal leaves have sheathing bases enclosing the stem apex, which is 

 moreover abortive. In fact no case has been produced in which the stem 

 apex is directly transformed into a terminal leaf and until this can be shown 

 we may continue to treat leaves as characteristically lateral organs. 



When we consider the phylogeny of the angiospermic leaf we find that it 

 shares with the pteridophvtic leaf a character which is of wide significance, 



