120 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, hi 



CELL-CONTENTS 



There are plants and parts of plants, including various Thallophyta 

 and spores of Cryptogamia, that can be killed by desiccation only with 

 very great difficulty, although they are quite devoid of any particular 

 morphological means of protection. This faculty is clearly correlated 

 with the nature of their habitat.^ The more striking features in this 

 connexion are treated in the succeeding paragraphs. 



Mucilage is common in mucilaginous cell- walls, or in the sap of cells 

 which are frequently large ; it absorbs water readily, but parts with it 

 very slowly, and is therefore manufactured by xerophytes in various 

 organs, including hairs, foliage-leaves, stems, subterranean tubers and 

 bulbs. There is a correlation between the production of mucilage-ceUs 

 inside the plant, and the development of the tegumentary tissue. Cacta- 

 ceae, such as Echinocactus, that have a well-developed hypoderma, 

 possess no mucilage-cells. The mucilage-cells of the Cactaceae are often 

 situated in the edges, the bosses, or other protuberant parts, which are 

 most exposed to drying.^ Possibly acting in the same way as mucilage, 

 there are other substances, such as 



Acids for instance, malic acid in Crassulaceae^ ; 



Tannin, which abounds in certain desert-plants * ; 



Salts, in halophytes ; 



Latex probably plays the same part.^ 



WATER-TISSUE 



Land-plants, particularly those exposed to strong transpiration, 

 develop specialized water-storing tissues. 



True water-tissue is thin-walled, contains water but no chlorophyll, 

 is devoid of intercellular spaces (as no gaseous interchange occurs in it), 

 and its cells are usually very large. It is capable of collapsing when 

 water is abstracted, and of expanding when the cells once more absorb 

 water. Water-storing tissue may be peripheral {epidermal or hypodermal) 

 or internal. 



Peripheral Water-tissue. 



The epidermis is the outermost layer acting (except in plants growing 

 in water or shade) as a water-tissue, as was indicated first by Pfitzer ^ 

 and subsequently by Vesque ' and Westermaier. This view is supported 

 by the facts that the epidermis usually contains no chlorophyll, that 

 it forms a continuous layer which in certain cases is very deep and is 

 directly connected with internal water-tissue, for instance in Velloziaceae.^ 

 The epidermis is specially differentiated in the Graminaceae, Cyperaceae, 

 and Velloziaceae, which have hinge-cells ^ along definite lines on the upper 

 face of the leaf, and especially in the furrows of the upper face ; these 

 cells are larger and much deeper than the other epidermal cells ; they 



' See G. Schroeder, 1886; V. B. Wittrock, 1891. ' Lauterbach, 1889. 



' G. Kraus, 1906 a. * Jonsson, 1902 ; Henslow, 1894. 



^ See p. 125. " Pfitzer, 1872. ' Vesque et Viet, 1881. 



Westermaier, 1884. ' For illustrations see Warming, 1893. 



" See p. 109. 



