CLIMBING AND WATER PLANTS 121 



has grown for a certain distance, it bends 

 up out of the mud into the air. Then as it 

 grows on, it curls down and its tip again 

 enters the mud. The bowed portion or 

 44 knee " which sticks up into the air forms 

 a quantity of spongy tissue full of intercellular 

 spaces, and as these communicate externally 

 with the atmosphere, and internally with 

 the intercellular spaces in the rest of the root, 

 the respiration of the root cells is amply 

 provided for, although there is no supply of 

 free oxygen in the mud through which they 

 grow. 



A number of other trees of the mangrove 

 swamp form special roots which grow up like 

 spikes out of the mud. They do not again 

 turn and grow downwards, but are definitely 

 specialised as aerating organs. They may be 

 compared to ventilating pipes, for their use 

 is entirely confined to enabling an interchange 

 to take place between the air in the plant and 

 that of the atmosphere. 



The submerged aquatic plants have to 

 meet a set of conditions very different from 

 those which confront the land vegetation. 

 Inasmuch as they are surrounded with water 

 there is no risk of desiccation, and the cuticle 

 is poorly developed and often is hardly 

 perceptible. Water is not continually being 

 lost, nor is there any difficulty in obtaining it. 

 Hence it is not surprising to find that the 

 development of water-conducting elements is 

 feeble. And one of the striking features of 



