THE ANGIOSPERMAE 



1599 



to grow, but if they fall into the water they may float out to sea and travel 

 great distances without losing their vitality. 



Another well-known case of vivipary is that of Cryptocoryne ciliata, a 

 small Aroid which grows in the brackish Nipa swamps of S.E. Asia. This 

 is a case of vivipary within the seed, for the seeds are shed and float in the 

 water as also do the embryos when liberated. The plumule forms a cluster 

 of about a score of primary leaves, enclosing two or three leaves of the 

 mature form (Fig. 1458). This emerges from the inner integument, only 

 the haustorial cotyledon remaining behind. The outer integument is 



Fig. 1458. — Cryptocoryne ciliata. A, Section of an 

 immature seed. The swollen cotyledon, below, 

 fills the embryo sac. The plumule is already 

 fully developed. B, Older embryo detached 

 from the cotyledon, the scar of which lies beside 

 the rudiment of the first root. {After Goebel.) 



gradually absorbed and its remaining outer layer distended by the plantlet 

 within. The hypocotyl becomes tuberous and finally the embryo breaks 

 away from the cotyledon, bursts the integument and falls out, opening 

 into a leaf rosette and forming roots as it floats. 



The unique morphological nature of the hypocotyl, in which stem and 

 root appear to meet and mingle, has long made it an object of interest. 

 After Naegeli pointed out, in 1858, the characteristic diflrerence in the 

 arrangement of the vascular tissues in the stem and in the root, the interest 

 began to centre upon what was regarded as a kind of puzzle, namely how 

 the two systems were joined up. The nature of the change and the trans- 

 ition level at which it occurred were sought and Van Tieghem in 1871 

 essayed to give precision to the study by defining three types of transitional 

 change. His descriptions treat the bundles topographically from below 

 upwards, the root structure being viewed as transforming itself into that 

 of the stem. 



Type I. The phloem bundles of the root run straight up into the 

 hypocotyl. The xylem bundles at the transition level increase in size and 

 divide radially. The halves diverge to right and left respectively and turn 



