AND HISTOLOGY OF PIPER BETLE. 381 
superficial tissues, and that in all cases they hasten to complete their 
development when the surrounding tissues are yet embryonie, in order to be 
able to extend protection to the latter when they need it most. 
Regarding mucilage-canals, it is noteworthy that they are absent from the 
root. [tis possible to infer from it, inter alia, that although the leaves are 
exposed to xerophytie surroundings the roots have not to deal with similar 
conditions, unlike those of a desert plant. Absence of lignification in the 
root to the extent observed in the stem points to the same inference. In the 
aerial parts the canals make their appearance very early. 1t is true they are 
absent from the blade (the solitary one in the midrib being dropped out of 
consideration) ; but it eould be explained by the development of. the water- 
jacket or aqueous tissue, which serves the purpose of mucilage, perhaps more 
efficiently. There is one more fact to notice regarding the distribution of 
these mucilage-canals. Though hardly represented in the blade, they are 
abundantly developed in the sheaths, which are obviously evolved for 
protection of the bud. 
The absence of an air-containing tomentum is fully explained by the 
evolution of the several complementary devices reviewed above. 
The hydathodes may next be noticed. They are known in plants to 
perform the double office of excretion as well as absorption of water. Their 
presence implies clearly that the plant is exposed to varying conditions of 
water-supply, now excessive, now deficient. 
One thing more regarding the leaf *. The entire and recurved margin 
is characteristic of xerophilous plants f, and implies that the plant once grew 
under conditions very different from those it finds to-day under cultivation 1. 
Strong and early lignification in the stem is one more physiological 
character that points in the same direction $. In certain cases more than 
half of the pith-parenchyma is found sclerosed. All the bundles are tipped 
with fibres at either end. The outer ring of vessels is bounded on both sides 
by well-developed sheaths of sclerenehyma. — Lignification also includes 
the broad medullary rays between peripheral bundles. Lastly, fibres are 
* Contrast the leaves of sclerophyllous plants (Schimper's * Plant Geography,’ pp. 510-12). 
t Warming, ‘(Ecology of Plants,’ Engl. ed. pp. 101 & 258. 
f In parts of Gujarat the vines are trained on a horizontal platform (5 feet above ground) 
of dry sticks facing towards the sky without any shade or cover. In other places, 
e.g. Ratnagiri District, the vines are trained along the stems of isolated trees. The leaves in 
both cases are smaller in size and thicker in texture than those produced in a regular grove 
in the Deccan. The comparatively large size of the leaf is thus probably a secondary 
character resulting, inter alia, from the great humidity that undoubtedly prevails in a grove 
for the growth of betel-vines. 
§ “ The physiological reason for the strong development of lignified constituents is still 
somewhat obscure. But intense light and vigorous transpiration seem to be the causes” 
(Warming, p. 129). 
