HYDROPHYTE SOCIETIES. 179 



be noticed that the habit of anchorage demands the 

 development of special organs of attachment, which usu- 

 ally take the form of root-like structures, often associated 

 with sucker-like disks. Associated with the anchoring 

 structures is often a development of floats, which is es- 

 pecially characteristic of seaweeds, enabling the working 

 body to float freely in the water (see Figs. 153, 154). It is 

 evident that while free-swimming forms may be suitable 

 for stagnant waters, anchored forms are better adapted for 

 moving waters. Therefore, where there are currents of 

 water, or wave action, the anchored forms predominate. 

 The ability to live in moving waters, and often in those 

 that become violently agitated, has its advantage to the 

 plant in the more rapidly renewed food material. In such 

 a situation free-swimming forms would soon be stranded 

 or disposed of in quieter waters. 



In the case of the marine seaweeds there is an interest- 

 ing relation between the depth of the water and the color 

 of the plants. While the fresh water algae are prevailingly 

 green, it will be remembered that the prevailing colors of 

 the alga3 of the seashore are brown and red. The brown 

 often passes into some shade of yellow, and the red may 

 merge into purple or violet, but in general the two types of 

 color may be called brown and red. It has been noticed 

 that the brown forms are found at less depth than the red 

 forms, so that in a general way there are two zones of dis- 

 tribution in relation to depth, the red zone being the lower 

 one and the yellow zone the upper. Just what this means 

 in the economy of the plants is not clear, but it has been 

 suggested that the yellow and the red colors assist the 

 chlorophyll in its work, which is more or less interfered 

 with by the diminished intensity of the lighl passing 

 through sea water. 



135. Loose soil societies. — This phrase is used merely to 

 contrast with rock societies, referring to the fact that the 

 anchorage is not merely for mechanical support, but that 



