AQUATIC PLANTS : CLIMBING PLANTS 295 



Hydrobryum (see Fig. 22). It drifts or is firmly fixed to 

 the substratum by fine hairs, and is at the same time the 

 assimilating organ of the plant. These plants are perfectly 

 suited for life in the most violent water currents. It may 

 be noted that in such water the air supply is very good, and, 

 in relation to this, the plants of these two families, unlike 

 most aquatics, possess practically no internal air spaces. 

 These remarkable plants have been investigated by Warming 

 ( 1 881) and Willis (1902) ; an account will also be found in 

 Arber's book. 



The mechanical tissue of the stem of aquatics is very 

 much reduced. The vascular bundles ^how only a few 

 vessels, and there is no sclerenchyma. The most prominent 

 anatomical feature is the abundance of air spaces. The 

 difference between typical land and water stems is well seen 

 by comparing the land and water forms of a single species 

 such as those of Polygonum amphihium. 



§ 6. Climbing Plants 



Not all land plants support unaided the weight of their 

 foliage. The ramblers, the root climbers, the twiners, 

 and the tendril climbers form a very large and varied 

 biological class. 



Ramblers. — By ramblers we mean such weak-stemmed 

 plants as the bramble or goose-grass, which scramble over 

 the surrounding vegetation. They have no special means 

 of climbing, though we may regard the thorns and prickles 

 which they often possess as enabling them to maintain a 

 position once gained. These may be single cells or large 

 epidermal and cortical emergences ; they are frequently 

 reversed or hooked, and so give a good purchase against 

 the downward pull of the shoot. 



Root Climbers.— We have an excellent example of a 

 root climber in our native ivy. The adventitious roots 

 are produced in great numbers on the unilluminated side 

 of the stem. They are ageotropic and negatively photo- 

 tropic. They penetrate cracks and crevices in the wall, 



