49 RELATION TO ENVIRONMENT. 



in comparison with the bulk of the plant, enable the plant to 

 resist long periods of excessive drought. In fact it is very diffi- 

 cult to dry out the stems of some of these plants provided with 

 water storage. 



952. Hydrophytic structures. From what we know of the 

 life of plants on the dry land it is evident that water plants have 

 some very different problems to solve in their life processes. 

 Some of these adaptations in structure are as follows: 



1. Provision for attachment. 



2. Provision for floating. 



3. Provision for aeration. 



4. Provision for distribution of food. 



5. Provision for fruiting. 



6. Provision for protection from water movements. 



1. Provision for attachment. In most of the aquatic flowering 

 plants and ferns attachment to the ground is accomplished as in 

 the case of land plants by roots. The roots here serve chiefly as 

 holdfasts, since there is only a slight development of root-hairs 

 or none at all in the case of submerged plants anchored in the 

 soil or rock crevices. Where the entire plant body or a large 

 part of it is immersed in water, absorption takes place through 

 the surfaces in touch with the water. The development of vas- 

 cular bundles is weak or wanting, since it is not necessary for the 

 plant to distribute water from the root system. See note, p. 712. 



2. Provision for floating. This is provided for by large inter- 

 cellular spaces which contain air or other gases. The air-spaces 

 in some stems and leaves are much greater than the bulk of the 

 tissues themselves, and serve to buoy up the plant. In the case 

 of many plants like the potamogetons, where the leaves are 

 entirely submerged, or some of them float on the surface of the 

 water, the stems are slender and pliant, and do not possess 

 strengthening tissues sufficient for support. The more or less 

 upright position of the stems is due to the floating power afforded 

 by the numerous large air-spaces. Oxygen given off by some 

 filamentous algae in the process of photosynthesis becomes caught 

 between the threads in a tangle and floats the plant on the sur- 



