CHAPTER XIII 



Anatojiy in Relation to the Habitat 



The fundamental organisation of the plant is essentially the 

 same for that of the desert as for that of the lake or mountain- 

 top, but the detailed structure is nevertheless subject to con- 

 siderable modifications in harmony with the differing conditions 

 of the environment. The most striking of these modifications 

 are related to the conditions of water-supply in the varied habitats 

 in which vegetation occurs, but light and other factors may also 

 play a part in moulding the structure of the plant. The effect 

 of diverse conditions is most patent when the self-same species 

 occupies two different habitats, as in the case of the sun- and 

 shade-forms of many common plants, and the land- and water- 

 forms of aquatics. 



The anatomical features exhibited by plants that have to 

 economise their water-supply ^ may be taken first. Such economy 

 may be necessitated by diverse factors, the most important of 

 which are deficiency of water in the soil [e.g. sand-dunes) , extreme 

 transpiration (as on a heath), or conditions which retard absorp- 

 tion by the roots (e.g. moorlands) ; these factors may either act 

 separately or several may operate simultaneously. Many of the 

 structural peculiarities, associated with environments in which 

 such factors prevail, are of the nature of transpiration-checks , 

 whilst others are connected with the storage of water during 

 times of plenty to be gradually utilised during periods of drought. 

 Among the former the most important are : development of a 

 thick cuticle, depression of the stomata below the general surface 

 (cf. p. 96) and other modifications of the stomata, restriction of 

 the latter to grooves or pits, copious production of hairs (p. loi), 

 and reduction of the leaf -surf ace. - 



' Such plants arc frequently spoken of as xerophytes. 

 ^ See also F. & S., chapter xiii. 

 158 



