How Plants Draw in Various Materials 195 



can grow in a soil that has no aeration. And it is interesting to 

 note, by the way, that many interesting accessory adaptions 

 are displayed by these plants, of which one in particular is here 

 apposite, viz., the walls of the air passages in these Water Plants 

 are so modified chemically that water will not wet them, and 

 therefore will not enter them by capillarity, on the principle dis- 

 cussed earlier in this chapter. This is obviously an advanta- 

 geous adaptation against the obstruction of these slender passages 

 by water in case of sub-aqueous accident to the petioles or stems. 

 In some other cases accessory aeration structures are developed 

 which permit a shorter route from the air to the roots. Of this a 

 conspicuous case has been claimed to exist in the great knees of 

 the Bald Cypress of the Southern swamps, which rise above the 

 water surface and contain an aeration system in connection with 

 the roots; and other comparable cases are known. In some 

 Water Plants, however, the aeration is of a simpler sort, con- 

 sisting indeed of an absorption of air dissolved in the water, in 

 precisely the manner used by the Fishes. In some kinds, for 

 example some Eel-grasses, the leaves are so thin as to present 

 a relatively great surface in proportion to the bulk of tissue to 

 be ae'rated; while in others the leaves are cut to the finest divi- 

 sions, presenting indeed a condition directly comparable physio- 

 logically with the gills of the fish. This is the reason for the tissue- 

 thin and thread-fine structure of practically all plants which live 

 wholly under water. 



Finally we must give some further attention to the particular 

 organs of absorption, the Roots. The structure of the young white 

 tips has already been described except for one point, viz., the 

 water-carrying ducts and the food-carrying sieve-tubes do not 

 stand in-and-out from one another as in young stems, but alter- 

 nately. In this arrangement lies an obvious adaptation, since 

 it removes the sieve-tubes out of the path of the water from 

 hair cells to ducts; and this conclusion receives some confirma- 

 tion from the further fact that the arrangement is not main- 



