CHAPTER XXII. 



HYDROPHYTES. 



444. IV. Hydrophytes may be divided into three groups : 

 i. Slime plants, which grow in the mud or slime at the bot- 

 tom of bodies of water. Here belong many algae, especially 

 diatoms, many species of low fungi, and bacteria in great 

 numbers. 2. Submersed plants, either free or attached. 

 Many algae, including both the diatoms and the filamentous 

 algae, are found floating in the water at various heights, 

 sometimes near the surface, sometimes more deeply submersed. 

 Since their substance is heavier than water, their capacity to 

 sustain themselves depends upon the production of gases in 

 the interior of the cells, or upon the presence of gases en- 

 tangled among their filaments. A few of the higher plants 

 are also found submerged and free, such as the bladder-worts. 

 The number of free-floating plants of the larger kinds is 

 small compared with those attached. The higher algae, 

 moss-worts, fern-worts, and seed plants are usually fastened 

 in the mud or to sticks and stones. The thallus of the algae 

 is usually profoundly branched and the shoots of the mosses 

 are richly supplied with leaves. All of the submerged fern- 

 worts and seed plants are characterized by a very thin -walled 

 epidermis, the absence of stomata, and the extensive surface 

 due to the very profuse branching of the stems or leaves, or 

 to the great number of these, or to both. In all cases the 

 extensive green surface may be looked upon as an adaptation 

 to securing carbon dioxide and the manufacture of sufficient 



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