172 



PLANT RELATIONS. 



work of conduction is not so extensive or definite^ and 

 therefore in such water plants the woody bundles are not 

 so i)rominently 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 {SargasRum), showing the thallus 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 t.<iking 



