150 COMPENDIUM OF GENERAL BOTANY. 



is only facultative.^ Conclusive results have not yet been obtained 

 in regard to other species. (It must also be borne in mind that 

 there are many investigators who deny tliat green plants can as- 

 similate animal food. — Tkans.) 



X. THE STOKING AND FUNCTION OF KESEKYE 

 MATERIAL. 



{a) Storing of Water. 



The epidermal water- supplying system ("aqueous tissue") 

 acquires such thickness in some plants {Pipeixicece., Bromeliacece) 

 that it evidently not only serves as a water bearing covering, but 

 also as a reservoir for water. Some internal aqueous tissues also 

 belong here. For example, in the leaf of some species of Aloe an 

 internal water-bearing tissue is enclosed by the assimilating tissue. 

 In such orchids as are especially adapted to withstand great dryness 

 isolated water-cells (idioplasts) are found distributed through the 

 assimilating tissue. These reservoir-tracheids have fibrous thicken- 

 ings of the M-all which prevent the collapse which would be caused 

 by the excessive hydrostatic pressure of the surrounding cells. 

 One of the ty]iical]y xerophilous plants, Mesemhnjanthemum. 

 crystalliiim., is supplied with enlarged epidermal cells occurring in 

 the leaf and petiole, which are filled with water. During excessive 

 dryness the plant receives its supply of water from these cells. In 

 some extreme cases of the development of water-tissue the water 

 contains a large percentage of saline substances in solution which 

 reduce transpiration. In xerophilous grasses {Eragrostis., Cyno- 

 don) it has been observed that the leaves become alternately broader 

 or narrower according to the amount of water present. This is 

 due to the fact that the lamellae of the water-tissue alternate with 

 the lamell?e of the assimilating tissue ; the former shrink on the 

 loss of water, thereby reducing the width of the leaf. In still other 

 cases there is a folding and unfolding due to similar changes within 

 the so-called "hinge-cells" (Tschikch). 



>Pfeffer, Uber fleiscbfressende Pflanzen, etc., LandwiitscliafUicbe Jabr- 

 bacber, 1877. 



