CHAP, xxxi ABSORPTION OF WATER BY LAND-PLANTS 119 



incrustation of salts as a coating that decreases transpiration, and suggests 

 that in this way the plants rid themselves of a portion of the salts absorbed; 

 and this view is adopted by Haberlandt. 1 



The velamen of Orchidaceae and Araceae has already been discussed. 2 

 In sundry epiphytic ferns and Araceae the aerial roots remain short, 

 grow more or less vertically, and collect among themselves humus, and 

 consequently wafer. 3 



Felted investments formed by roots or remnants of leaves, or both, 

 occur in ferns such as Dicksonia antarctica, species of Alsophila, as well 

 as in Velloziaceae, and palms. Some of the plants concerned are obvi- 

 ously xerophytes, and the investment serves not only as a means of 

 protection against transpiration, but also as a device for collecting and 

 storing water. 4 According to Buchenau the same is true of the juncaceous 

 Prionium serratum (P. Palmita), which grows in the periodically dry 

 river-beds of South Africa. In this category must also be placed the 

 grasses that Hackel 5 terms tunic-grasses, which retain water between 

 the macerated or scale-like persistent leaf -sheaths. 



To this group of devices for the obtaining of water may be added the 

 felted mass of rhizoids of many mosses. Many arenicolous xerophytes, 

 especially arenicolous grasses, form dense tussocks or cushions, which 

 certainly benefit them by collecting and retaining water. 



Other organs, leaves for example, may likewise be designed to take 

 up rain and dew. In such cases the leaves are usually more or less 

 trough-like or, as in Umbelliferae, provided with large sheaths ; as marked 

 examples may be cited the majority of Bromeliaceae, Pandanaceae, and 

 the sugar-cane ; a specially remarkable form is Tillandsia bulbosa, whose 

 narrow trough-like leaves easily obtain water and convey it to the cavities 

 between the inflated leaf -bases. 6 



Particular forms of leaves fitted to take up and hold water are possessed 

 by many epiphytic liverworts. Of these Gobel 7 distinguishes three 

 types, according as the under-lobes of the leaves alone or with the co- 

 operation of the upper-lobes form water-reservoirs, or as the leaves form 

 peculiar bowl-like water-sacs. 



CHAPTER XXXII. STORAGE OF WATER BY LAND-PLANTS. 

 WATER-RESERVOIRS 



A VERY important and common device, by which the land-plant is 

 enabled to endure dryness of air and soil, is the construction of tissues 

 or organs capable of conserving water for use when it shall be required 

 for purposes of assimilation and other functions. This type of device 

 is frequent among xerophytes, but completely lacking in hydro- 

 phytes. 



1 Haberlandt, 1904; see also Job. Schmidt, 1903. 2 See p. 104. 



3 Gobel, 1891-2 ; Karsten, 1894. * Warming, 1893. 



6 Hackel, 1890 ; also see figure in Warming, 1892. 

 8 Schimper, 1884, and 18880. 7 Gobel, 1898-1901, vol. ii, p. 58. 



