480 



In addition to thus protecting themselves against excessive trans- 

 piration, plants inhabiting dry regions have generally a very well 

 developed root system, the roots sinking into the soil to a great depth 

 in search of moisture : they also exhibit various other modifications 

 in their structure. Perhaps the most important of these modifica- 

 tions, is the development of water storing cells, which have the 

 power of storing up water when the supply is temporarily increased, 

 and yielding it up to the growing tissue when the supply from other 

 sources falls short of the demand made upon it by the plant The 

 leaves of Agaves, Fourcroyas and Sansevierias ; the stems of Cacti 

 and the pseudobulbs of epiphytic orchids, are chiefly made up of 

 this aqueous tissue ; the cells of which being gorged with water, 

 constitute a reservoir from which the loss of water due to transpira- 

 tion is made good. Owing to the presence of this water storing 

 tissue, these plants can withstand long periods of drought without 

 injury ; the cut branches of Cacti for example, will remain alive for 

 weeks or months, even when exposed to the sun, before the stored 

 up water becomes exhausted. 



A familiar example of a plant with water storage tissue, occurs in 

 the case of the Purslane {"Gelang Pasir "), an exceedingly common, 

 although harmless, weed on most estates, especially in the neighbour- 

 hood of cooly lines. It has small fleshy leaves which are used as a 

 vegetable by the coolies, and bears little yellow flowers. The 

 difficulty of eradicating this plant is known only too well to all 

 planters, as, thanks to its special provision of water, each small piece 

 of the plant allowed to remain on the ground after hoeing out the 

 weeds, is generally able to re-establish itself before becoming 

 desiccated : in fact, cultivation of the soil rather assists in spreading 

 the plant than otherwise 



As a rule, when the leaf surface is much reduced, the plant is 

 armed with thorns : the terrible spines developed on Cacti and other 

 xerophilous plants, serve as a protection against the depredations of 

 herbivorous animals. 



Xerophilous plants are of course most perfectly developed in hot 

 deserts as for instance in parts of the United States, Mexico, the 

 Sahara and parts of Australia ; but also occur in the Artie regions 

 (where absorption is limited by the prevailing low temperature) and in 

 salt marshes. It may seem strange that the plants which form the man- 

 grove swamps at the mouths of rivers, and on the sea shore, where 

 there is an unlimited supply of water, should exhibit the same struc- 

 tural peculiarities as xerophilous or dry region plants : yet an 

 examination of their organs show that they too have developed thick 

 leaves, sometimes covered with hairs, are provided with water stor- 

 ing tissue, and in many respects agree with true xerophytes. The 

 explanation of this is doubtless to be found in the fact that such 

 plants find a difficulty in separating the water from the salt solu- 

 tion, and if free transpiration occurred, it would result in an ac- 

 cumulation of salt within the tissues of the plant which would prove 

 injurious. A similar structure is characteristic of the plants in- 

 habiting peat bogs, where the presence of large quantities of humous 



