CH. IV] MORPHOLOGY. 53 



have any embryonic tissue, in other words, all the cells 

 which compose the leaf soon take on a permanent 

 character. Compare for instance the leaf of an oak 

 and the branch that developes from the bud in its axil ; 

 the leaf increases until it attains a length of 2 inches 

 or so, and then it grows no more, but the branch grows 

 year after year and continues to bear, in the buds which 

 cover it, innumerable growing points. 



Another point is of importance; the flowers, which 

 are the reproductive parts of a plant, are borne on 

 branches, whereas the leaves do not bear flowers. This 

 may be illustrated again by the Butcher's Broom, whose 

 flowers grow on the flattened leaf-like branches above 

 described. The position in which the flower is borne 

 has also a wider morphological importance. It is one of 

 the characters that distinguish the whole of the root- 

 system from that of the stem. At first sight it seems 

 absurd to appeal to such a character to mark off root 

 from stem, since a typical stem and a typical root are 

 so different in appearance. But in the potato an under- 

 ground rootlike stem has been met with, and in the fern 

 another instance will occur. Roots, on the other hand, 

 are by no means always underground : the aerial roots of 

 the ivy have been described, and many such cases occur ; 

 we must therefore look for more fundamental distinctions. 

 It has been seen that a stem, the potato-tuber, may be 

 devoid of chlorophyll, but it might have been hoped, 

 when a part of a plant is found to be green, that then at 

 least we should know it not to be a root. But certain 

 tropical orchids have flat green aerial roots which actually 



