WATER-TISSUES 397 



their very existence upon the possession of special water-reservoirs. 

 " Water-tissues," and the other histological arrangements which sub- 

 serve a similar purpose, are usually located in the immediate neighbour- 

 hood of the photosynthetic system, because this system is more liable 

 than any other to suffer injury through failure of the water-supply. 

 On the other hand, provision must also be made for the ready access 

 of water from the conducting channels to partially or entirely depleted 

 reservoirs. 



After these preliminary remarks, we may proceed to characterise 

 and define the storage system somewhat more formally. The storage 

 system, then, may be said to comprise, in the first place, all those 

 tissues which have as their principal function the storage of synthetic 

 products, that will subsequently be utilised in connection with growth, 

 or for other physiological or ecological purposes ; it further includes 

 the various structures which serve as reservoirs of water. 



In view of the great diversity of reserve-materials, but little can 

 be said concerning the general character of the storage system. Most 

 storage-tissues, however, are, in accordance with their function, mainly 

 composed of large-celled parenchyma ; in addition they often display 

 various features which tend to accelerate the entrance and removal of 

 the reserve-materials. 



II. THE STORAGE OF WATER 



A. WATER-TISSUES. ^ 



In many plants of warm climates the uninterrupted activity of the 

 photosynthetic organs is ensured by the presence of layers of water- 

 storing cells, which constitute the so-called water-tissue of Pfitzer. 

 This specialisation is particularly characteristic of xerophilous species 

 that grow on rocky ground, or which lead an epiphytic existence, such 

 as the Bromeliaceae, many Orchidaceae, and the members of genera 

 like Pcperomia, Begonia, and Tradescantia. In the tropics, the need 

 for water-storing arrangements of this kind may even be felt by hygro- 

 philous plants, especially if their leaves are large and delicate, in which 

 case transpiration may at times attain to enormous proportions under 

 the influence of the prevailing intense insolation. This condition is 

 exemplified by Oanna, Maranta, and other genera of Scitamineae, by 

 Ficus elastica, Conocephcdus ovatus, Euphorbia thymifolia, and by many 

 Palms. Many littoral plants, including the semi-aquatic Mangroves, 

 are likewise provided with well-developed water-tissues. In these 

 latter cases, the succulence of the leaves is regarded by Schimper as a 

 safeguard against excessive loss of water by transpiration and conse- 

 quent diminution of photosynthetic activity owing to accumulation of 



