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 
*“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 
