HOLE : SOME INDIAN GRASSES AND THEIR (ECOLOGY. 7 



The supply of oxygen can also be augmented by increasing the absorbing surface of 

 the plant which is in contact with the water and hence submerged leaves and, in some 

 algae, the entire plant are often cut into a number of long narrow segments. 



Oxygen being usually more plentiful in running than in stagnant water, plants in 

 running water tend to show comparatively less development of aerating devices. 



The presence of aerating tissue is also of utility in increasing the buoyancy of the 

 plant. 



In woody marsh plants the so-called pneumatophores, or aerating roots, are often 

 developed as an adaptation to meet the danger of a deficient oxygen-supply. 



To some extent also the characteristics of the plants of this group are due to the 

 weakening of the light in water and submerged parts of stems and leaves tend to become 

 long and thin resembling those of terrestrial plants developed in shade. 



The epidermis, especially of submerged parts, as a rule exhibits no devices for reduc- 

 ing transpiration such as a thick cuticle, covering of hair, wax, etc., and hydrophytes, when 

 entirely removed from water, usually dry and wither with great rapidity. 



As examples of this class of plants we may take aquatic Algae or " sea-weeds." 



Trapa bispinosa, the water Chestnut, with its remarkably branched submerged 

 leaves with filiform segments. 



Nelumbium speciosum, the Sacred Lotus, with the enormous air spaces in its sub- 

 merged creeping stem. 



Xerophytes. 



The root system is usually strongly developed. Adaptations for reducing transpira- 

 tion are very common and characteristic and amongst the most usual devices resorted to 

 are — 



(1) A reduction in the surface of those members which are, as a rule, most special- 



ised with the object of facilitating transpiration, viz. the leaves. Thus 

 some xerophytes are characterised by the relatively small size of their leaves 

 and leaflets and others dispense more or less completely with leaves and place 

 their chlorophyll tissue in their stems or branches. 



(2) Development of a thick protective cuticle and often also of thick walled scleren- 



chymatous tissue beneath the epidermis wh>ich interfere with the free exchange 

 of gases between the outer air and the moist thin-walled chlorophyll tissue 

 beneath them. Coatings of wax and hairs serve the same purpose. The 

 latter can as a rule be easily recognised by eye and the former anatomical 

 characters are frequently indicated in the field by the firm hard texture and 

 also by the colour of the foliage, the colour of those leaves in which the chloro- 

 phyll is protected by thick-walled external cells usually being like that of 

 leaves which are covered with a thin coating of wax, viz. a dull, or bluish- 

 green, as contrasted with a fresh bright green. 



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