THE AXGIOSPERMAE : LEAVES 



975 



roots, which are absent, are taken over by long thread-Hke leaves, the rhizo- 

 phylls, which have a downwardly directed and indefinite apical growth. 

 Only their origin and the occasional occurrence of imperfect insect traps on 

 the rhizophylls remain to show that they have originated from modified 

 leaves and are not, in fact, as in appearance, true roots. In some species of 

 Utricidaria the persistence of apical growth in the leaves has led to a situation 

 where all distinction of stem, root, and leaf appear to be lost. The vegetative 

 plant consists of creeping axes which may develop either from or into foliage 

 leaves or into rhizophylls with complete promiscuity (Fig. 966). In this 



^""15^ 



Fig. 966. — Utricidaria coenica. Flowering shoot showing promiscuous de\elop- 

 ment of branches into either leaves, roots or flowers. {After Goebel.) 



connection one mav recall that the leaves of Wehcitschia are also of unlimited 

 growth, though in their case the growth is intercalary. 



During the brief period of normal apical growth in the leaf rudiment, 

 the basal portion expands laterally, in accordance with the transverse expansion 

 of the meristem of the stem apex, so that it occupies an increased arc of the 

 circumference of the apical dome. The leaf rudiment in Monocotyledons 

 expands laterally so much that it rapidly encircles the whole growing point 

 like a collar, and the close succession of these collars, each growing actively, 

 quickly pushes the leaf bases of older rudiments centrifugally away from the 

 growing point. This produces the sharp flexure of the leaf-trace strands at 

 the point of their entry into the leaf base, which we have commented upon 

 in the previous chapter, as characteristic of most monocotyledonous stems. 



In describing differentiation at the apex of the stem, in Chapter XXI, 



