i 7 8 



BOG-PLANTS 



which is, however, probably an outcome of complex causes. 

 Excellent examples are furnished by the Cotton Grass (Erio- 

 phorum), the Rush (cf. p. 165), and the Bulrush (Scirpus), all 

 typical of such localities. The principal feature reminiscent of 

 aquatics is the vast system of intercellular air-canals which 

 serves to supply the underground organs with oxygen. 



The numerous examples, cited in this chapter have shown 

 that specialised structure and habitat often go hand in hand. 

 Thus the plants of a salt-marsh are mostly succulents, those 

 of a desert often have small leaves or assimilating stems, many 

 heath-plants have rolled or hairy leaves, whilst those of a wood 



FIG. 95. Transverse section of Clover-stern (Cl.) showing penetrating 

 haustoria (h.) of Dodder (Cuscuta) (Cu.), whose stem is cut longi- 

 tudinally. V., vascular tissue of Dodder. 



mostly possess large thin leaf-blades without marked checks to 

 transpiration. Certain types of structure are, moreover, asso- 

 ciated with certain families, as evidenced by the succulence of 

 most members of the Stonecrop-family (Crassulaceae) and Cactus- 

 family (Cactaceae) , the small-leaved habit of most plants of the 

 Heather-family (Ericaceae), and the aquatic character of all 

 members of the Pondweed-family (Potamogetonacese) . 



Modifications of structure in relation to the special mode of 

 life are also exhibited by the parasites and saprophytes amongst 

 Flowering Plants. 1 A striking feature of both is the feeble 

 development of the xylem, no doubt in relation to the reduc- 

 tion of the leaves (e.g. in the parasitic Dodder and in the 



1 Cf. F. & S., pp. 139 et seq. 



