98 



A PRIMER OF BIOLOGY 



more, delicate-stemmed plants are able to maintain 

 themselves in the erect position by holding on to 

 their stronger neighbours, and so enjoy the maximum 

 of air and light they would otherwise fail to obtain. 

 This they do by means of a variety of structures, 

 all of them performing the same function, but of 

 the most diverse morphological value. Cobsea, 

 for example, climbs by means of tendrils which are 



the terminal leaflets of 

 branched leaves (Fig. 25). 

 The grape also possesses 

 tendrils which perform 

 the same function, but, 

 in this case, the tendrils 

 are modified flower 

 branches. The bramble 

 climbs by means of 

 prickles, which are, at 

 the same time, protec- 

 tive, and the ivy by 

 throwing out aerial roots 

 which cling to walls, 

 trees, &c. 



A considerable amount 

 of experimental work has 

 been carried out of recent years on several plants 

 with the object of determining how far they may 

 be made to adapt themselves to changed surroundings. 

 Take, for example, the common gorse, familiar to 

 every one by its bright yellow flowers, spiny green 

 shoots and absence of genuine photosynthetic leaves. 

 If a seedling gorse plant be examined, it will be 

 found that it has no spines, but, on the other hand, 

 possesses branches with small but quite recognisable 

 leaves (Fig. 47). On cultivating such a seedling in 



FIG. 47. Ulex europseus: A, 

 grown in moist air ; B, grown 

 in dry air (1 nat. size). 



