

BOTANY OK TIIK LIVING PLANT 



lr 



is radially constructed, but the sympodial system is dorsiventral. 

 ig. 139.) Inothen the mature shoot may be itself dorsiventral, 

 though that condition is only acquired in the course of the individual 

 development, and is not shown by the apical bud. This is seen in 

 the rhizomes of Acorns, where the apical bud shows a bilateral sym- 

 metry, with alternate leaves. But in the mature rhizome the two 

 mw shifted to the upper surface, giving a dorsiventral condition, 



with roots arising from the lower surface. In other cases again, and 



conspicuously in Ferns, the dorsi- 

 ventrality is fixed already at the 

 growing point, and is not the 

 mere result of any subsequent 

 displacement of parts. This is 

 seen in the Common Polypody, 

 which has from the first distichous 

 leaves placed obliquely on the 

 upper surface of the rhizome, 

 while roots only arise on the lower 

 side. This arrangement is already 

 apparent at the growing point 

 (Fig. 140). 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, 

 e.g. in many grasses, such as Dactylis. But, as we shall see later, it is in the 

 flower that dorsiventrality becomes most marked, and of great biological 

 importance. In all cases among the Higher Plants the dorsiventral is probably 

 a condition which has been derivative from a primitive radial state. 



Fie. 140. 



PolypoJium vul^are. ( x 6.) Median section 

 through prothallus and embryo, showing one 

 series only of the distichous leaves I|, l u / s , 1-. 

 R — Tools, ap — apex of axis. The youngshoot 

 becomes inverted, growing backwards over the 

 prothallus. (F.O.B.) 



Plant Communities. 



It is thus seen that Climate and other conditions of life are frequently 

 related to special modifications in the plant, which meet special 

 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 adaptations. 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 



