242 ABSORBING SYSTEM 



dissolved by treatment with concentrated sulphuric acid. Accord- 

 ing to Mez, they consist of a cellulose matrix impregnated with a 

 high percentage of pectic compounds. The cuticle itself may also be 

 soluble in sulphuric acid (Vriesea psittacina), or it may persist after 

 treatment with this reagent in the form of an extremely delicate 

 pellicle {Tillandsia usneoides). 



When a Bromeliaceous leaf which is provided with a dense covering 

 of peltate scales is moistened, water is rapidly absorbed, and the 

 leaf, which was whitish in the dry state, assumes a greenish coloration. 

 Microscopic examination shows that the cells of the scales have 

 become filled with water. If a drop of caustic potash be placed upon 

 the epidermis and wiped off after a few seconds, it will be found that, 

 around each scale, the previously colourless contents of the epidermal 

 cells have assumed a golden-yellow tint. After treatment with salt 

 solution, the plasmolytic contraction of the protoplasts first appears in 

 the parenchymatous cells surrounding the base of each scale. All these 

 facts, taken in conjunction with the circumstance that the epiphytic 

 Bromeliaceae are either altogether rootless or at most possess only 

 grasping roots, and nevertheless remain fresh for weeks at a time, pro- 

 vided that their leaves are occasionally moistened, indicate that the scales 

 are extremely well fitted for the task of absorbing water and aqueous 

 nutrient solutions. 



A few words must finally be devoted to the function of the 

 thickened outer walls of the disc. In the dry condition of the hair 

 the thin lateral walls of the disc-cells are crumpled up, while 

 the thick outer walls almost touch the living stalk-cells, which are 

 sheltered by them as by a lid. On the access of water, the lateral 

 walls expand once more to their former size, and the lid is 

 raised. The disc thus represents a protective device, which prevents 

 the escape of water-vapour through the thin-walled stalk-cells during 

 periods of prolonged drought. 125 The action of the lid is very clearly 

 seen in the case of Vriesea psittacina. Here the circular disc (Fig. 93 a) 

 comprises, first, a number of radially elongated marginal cells, secondly, 

 a circle of eight tangentially elongated intermediate cells, and, thirdly, 

 a group of four central elements placed cross-wise. It is only the 

 central cells that have thick outer walls, and thus form the actual lid 

 (Fig. 93 b). When transpiration has been going on actively for some 

 time, the lid sinks down, until its margin rests upon the upper edge of 

 the funnel-shaped stalk, which thus becomes completely shut off from 

 the outer atmosphere (Fig. 93 c). 



Water-absorbing trichomes in general are, with few exceptions, 

 covered with a cuticle insoluble in sulphuric acid, even at the points 

 through which water actually enters. Whether the undoubted per- 



