172 



PLANT EELATIONS. 



work of conduction is not so extensive or definite, and 

 therefore in such water plants the woody bundles are not 

 so prominently developed as in land plants. 



(4) Reduction of mechanical tissues. — In the case of 

 ordinary land plants, certain firm tissues are developed so 



Fig. 154. Gulfweed (Sargassutri), showing the thalluB differentiated into stem-like and 

 leaf -like portions, and also the bladder-like floats. — After Bennett and Murray. 



that the plant may maintain its form. These supporting 

 tissues reach their culmination in such forms as trees, 

 where massive bodies are able to stand upright. It is evi- 

 dent that in the water there is no such need for rigid sup- 

 porting tissues, as the buoyant power of water helps to 

 support the plant. This fact may be illustrated by taking 



