400 STORAGE SYSTEM 



that the greater thickness of the old leaves is, in this case also, entirely 

 due to enlargement of the water-tissue. 



Thickness of the photo-synthetic tissue (includ- 

 ing the lower epidermis .... 



Thickness of the water-tissue (including the 

 upper epidermis) - 



Total thickness of the leaf - - 



If, moreover, a severed shoot of Rhizophora mucronata is left to 

 transpire without any external supply of water, the old leaves contract 

 very noticeably and acquire a wrinkled surface in the course of a few 

 days ; the turgidity of the younger leaves, on the other hand except- 

 ing those very young ones which are not yet fully developed is 

 scarcely affected, and their surfaces consequently remain perfectly 

 smooth. The result of this experiment indicates very clearly that the 

 old leaves act principally as reservoirs of water. 



Water-tissues are by no means restricted to leaf-blades and other 

 photosynthetic organs. In many epiphytic orchids, for example, storage 

 of water takes place in " pseudo-bulbs," which may consist of one or 

 several internodes. Water-storing tubers of varying dimensions are also 

 found among the Hnbiaceae, Vacciniaceae and Melastomaceae, and in 

 species of Gesnera. One of the most familiar examples of a water- 

 storing tuber is the common potato ; this instance may also serve to 

 illustrate the fact that in tubers as in bulbs both water and plastic- 

 materials are generally stored in one and the same tissue. 



The largest known water-storing organs are represented by the 

 fleshy tuberous stems of Hydnophytum and Myrmecodia , two myrmeco- 

 philous and epiphytic genera of Bubiaccae, which inhabit the Malay 

 Archipelago, New Guinea and Southern Australia. In these plants 

 the tuber consists of the enlarged hypocotyl. In Myrmecodia it is 

 about 30 cm. long and 20 cm. thick, while in Hydnophytum tortuosum, 

 according to Beccari, it may attain a diameter of 60 cm. It consists 

 of succulent water-tissue traversed by a complicated system of cavities 

 and passages which are all continuous with one another. In the case 

 of Myrmecodia this system communicates with the exterior through a 

 large aperture situated among the roots on the side next the sub- 

 stratum ; smaller lateral openings are also present. The walls of the 

 cavities are clothed, like the outer surface of the tuber, with a layer of 

 cork, which is covered with numerous whitish tubercles, interpreted 

 by Treub as organs of ventilation (lenticels). The cavities are occupied 

 by hordes of ants which strongly resent any intrusion upon the privacy 

 of their dwelling-place ; it is, however, difficult to imagine what enemies 

 of the plant could be kept at bay by these guardians, and there seems, 



