STRUCTURE. 47 



In waterlilies which are adapted to enduring a dry season, the 

 resting tubers differ markedly from the vegetative caudex. In the case 

 of young seedling plants, which alone form resting tubers in most 

 apocarpous species, the tuber acquires a smooth, corky bark on its 

 lower parts. The leaf and root scars are obliterated by intrusion of 

 cortical cells, which absorb the spiral thickenings of the xylem (Caspary, 

 1859) and give rise to several layers of typical cork. Although I have 

 never found these in section to have the regularity of arrangement 

 shown in some trees with a well-developed cork cambium (Ginkgo), 

 I have often with my hands peeled off the bark from a living tuber, 

 and found it to separate as smoothly as the bark of a willow twig in 

 April. The interior tissues are dense and hard with their load of starch, 

 and the vascular system remains intact except in its peripheral parts. 

 The irregular lateral tubers produced upon the sides of large plants 

 of the Lotos group seem to possess much less vascular tissue than the 

 vegetative caudex and to have much more cortex, 

 heavily laden with starch. The lacunar cortex is wholly 

 absent from resting tubers, probably because, as in N. 

 flavo-virens, the cork layer is formed inside the lacunar 

 tissue, and all of this region dries or decays away. 



T T^ i- i /i i FIG. 19. Cork forma- 



ln hu-castaha and Lnamaenymphaea, subenzation tion at base of root seari 

 takes place only over the scars of petioles, peduncles N - alba ' m < M *- 

 and roots, and on the boundary of the living and dead tissues at the old 

 decaying end of the stem. The root scars are healed over by a curved 

 surface, concave on the outer side, in the middle of which the vascular 

 bundle projects slightly. The cortical cells along this concave surface (which 

 lies four or five cells farther in than the proximal ends of the air-canals of the 

 root) undergo one to three parallel divisions (Fig. 19) to form the cork layer, 

 and not until some time afterward do the adjoining root tissues decay away. 

 I do not know how the vascular bundle of the root is closed. In A r . alba 

 candidissima, where the above facts were observed, the old rhizome, 

 decaying at its posterior end, seems to have its epidermis shriveled 

 and more or less disintegrated and its exo-cortex suberized in its cell- 

 walls, but usually without cell-division. When this layer peels off, 

 carrying with it the leaf scars, a rounded corky invagination or pouch 

 is seen (Fig. 20 K] at the base of each main air-canal of the petioles 

 and peduncles. The largest pouches extend inward a quarter of an 

 inch, and point slightly backward ; they are hollow, and wide open to 

 the exterior, and are usually full of mud. The whole surface of the 



