42 THE CELL 



of turgescent cells and tissues contributes very appreciably to the 

 mechanical strength of the plant-body ; turgor may even be the sole 

 source of mechanical strength, for instance in the case of small organs 

 which are devoid of special strengthening cells, or where the entire plant 

 is minute. It is by means of turgor-pressure that a growing plant over- 

 comes internal and external resistances, while variations of turgor play 

 an essential part in the execution of many paratonic movements. 



Cell-sap also frequently contains reserve-materials. In the case of 

 specialised water-tissues the water that forms the bulk of the cell- 

 sap itself represents a store which is drawn upon by other living 

 tissues, and in particular by the photosynthetic cells, in time of drought. 

 Among the plastic materials that is to say, such substances as are 

 subsequently used up in connection with growth-processes which are 

 commonly contained in cell-sap, the carbohydrates [sugars and inuline) 

 are the most important, but proteins and the amide asparagine are also 

 of frequent occurrence. The so-called aleurone-grains, for example, 

 which are often found in the storage tissues of seeds, are produced by 

 the drying up of protein-containing vacuoles. Varying amounts of 

 inorganic nutrient salts in the form of nitrates, sulphates and phosphates 

 may also be stored in the cell-sap. On the other hand, vacuoles not 

 infrequently contain various by-products of metabolism commonly 

 termed excretory substances such as alkaloids, as well as certain com- 

 pounds, the physiological value of which is as yet uncertain, namely, 

 tannins, glycosides, and so forth. Both these bodies of obscure signi- 

 ficance, and the definitely excretory waste-products, may find an 

 ecological application as instruments of protection against the attacks 

 of animals. Substances dissolved in the cell-sap may also be utilised 

 for other ecological purposes. In many flowers and fruits the presence 

 of the pigment anthoeganin, forming a red solution in acid and a blue 

 one in alkaline cell-sap, helps to attract pollinating or seed-dispersing 

 animals ; in the case of fruits the sugar contained in the cell-sap per- 

 forms a similar attractive function. Anthocyanin is also widely dis- 

 tributed as a constituent of the cell-sap in vegetative organs, especially 

 in leaves, where it in many cases probably acts as a light-screen which 

 prevents excessive illumination. 



Sometimes the vacuoles contained in a protoplast are not all of the 

 same nature, but differ from one another in the chemical composition 

 of their contents or in other particulars. Thus one may find vacuoles 

 containing coloured cell-sap side by side with others of which the 

 contents are colourless ; similarly, vacuoles containing tannin occur in 

 company with others which are devoid of this substance, and so forth. 

 In this connection reference must be made to the physodes described 

 by Crato, peculiar vacuoles attaining their greatest development in 



