PERIPHERAL WATER-TISSUES 



401 



therefore, little or no justification for the widespread assumption that 

 the plants in question are "niyrmecophytes." Treub has, in fact, shown 

 by means of culture-experiments, that both the tubers, and the cavities 

 and passages which they contain, develop quite normally in the entire 

 absence of ants ; he therefore quite properly regards these remarkable 

 organs as enormous water-reservoirs, and interprets the system of 

 passages as an arrangement for providing the massive and constantly 

 growing structure with a sufficiency of 

 oxygen [for purposes of respiration]. 



The physiological significance of 

 water-tissues was recognised long ago 

 by Pfitzer. Westermaier was, however, 

 the first to approach the subject from 

 the experimental point of view. The 

 three succeeding paragraphs reproduce 

 the principal results of Westermaier's 

 researches, and at the same time sum- 

 marise the leading physiological features 

 of epidermal water-tissues in general. 



1. If a leaf provided with a water- 

 tissue is allowed to dry up gradually, 

 the effects of the loss of water first 

 become apparent in the water- storing 

 cells themselves. These are found to 

 have undergone a considerable amount 

 of contraction, at a time when the 

 photosynthetic tissue shows little or no 

 signs of a shortage of water. This 

 faculty of rendering up its store of 

 water with ease and celerity is, of course, 

 the most essential condition of efficiency 

 in the case of any water-storing tissue. 



2. As water is withdrawn from 

 the storage-cells, they gradually con- 

 tract and collapse, and their thin radial walls are thrown into folds 

 or undulations (Fig. 159 b). This change of form is connected with 

 the circumstance that the water lost is not replaced by air; the 

 entrance of air would indeed be incompatible with the persistence of a 

 living peripheral layer of protoplasm in the cells. The unthickened 

 condition of the radial walls, which admits of this bellows-like contrac- 

 tion and expansion of the entire water-tissue, is thus seen to be an 

 anatomical feature of the first importance. 



3. When the contracted water- tissue is once more plentifully 



2c 



Fig. 159. 



T.S. through leaves of Peperomia tricho- 

 carpa. A. Leaf in the fresh condition. 

 B. Severed leaf, which has been trans- 

 piring for four days at lS-20 C ; ir, 

 water-tissue ; a, photosynthetic tissue ; 

 s, spongy mesophyll. 



