MODIFICATIONS OF FORM 



175 



roots only arise on the lower side. This arrangement is already 

 apparent at the growing point (Fig. 131). Such examples show 

 various steps in the impress of dorsiventrality on the vegetative 

 shoot. Lopsidedness may also appear in the inflorescence of many 

 flowering plants. But, as we shall 

 see later, it is in the Flower itself 

 that it becomes most marked, and 

 of great biological importance. In 

 all cases among the Higher Plants 

 the dorsiventral is probably a con- 

 dition which has been derivative 

 from the primitive radial state. 



Fig. 131. 

 Polypodium vulgare. ( x 6.) Median section 

 through prothallus and embryo, showing one 

 series only of the distichous leaves li, /..;, /.:;, /;. 

 /?=roots. a/> = apex of axis. The young siioot 

 becomes inverted, growing backwards over the 

 prothallus. 



Plant Communities. 



It is thus seen that Climate and 

 other conditions of life are fre- 

 quently related to special modifi- 

 cations in the plant, which meet 

 special needs. As the conditions 

 will be substantially similar for all plants which grow in a district 

 or specific area, they may collectively take characters which they 

 share in common. Thus they form characteristic Communities. 

 The water-relation more than anything else determines such adapta- 

 tions. Where the climate or conditions are dry, so that water 

 must be carefully conserved, the plants so adapted are termed 

 Xerophytes. Where water is abundant, or the vegetation is actually 

 submerged, the plants specially developed under such conditions 

 are termed Hydrophytes. Where the water is brackish or salt, plants 

 assume the characters of Halophytes. But where the conditiors 

 as regards temperature and water-supply are not extreme, though 

 there may be marked seasons, the vegetation would be described as 

 consisting of Mesophytes. Such groupings cannot be drawn with 

 definiteness. They must be held as generally descriptive rather than 

 mutually exclusive. In point of fact the communities overlap and 

 graduate one into another. Naturally the distinctive characters im- 

 pressed upon each community are best illustrated by extreme types. 

 Xerophyte Vegetation is characterised by various features which 

 have the effect, individually or collectively, of controlling transpira- 

 tion, and thus making the best of a limited water-supply. The leaf- 

 area is reduced, and its texture is often fleshy, so as to serve for 



