SUMMARY OF THE THALLOPHYTES 305 



of form is very great, but the highest types in the brown and 

 red algse are differentiated into stems, leaf-like structures, and 

 holdfasts. The cell structure of the thallophytes is generally 

 much simpler than that in the plant bodies of the bryophytes, 

 which owe their complexities of cell structure chiefly to the 

 varied conditions introduced by the land habit; for the land 

 habit requires the plant to protect itself from drying up, in 

 the air. This, in general, means that a land plant must obtain 

 water from the soil through some kind of organs adapted for that 

 purpose (rhizoids or roots). And, as a rule, a land plant soon 

 differentiates a protective layer of cells (epidermis), which helps 

 to hold the water within its tissues. These structures are either 

 entirely absent or present in greatly reduced form in aquatic 

 plants, and for these reasons the cell structure of the aquatic 

 thallophytes is generally very much simpler than that of the 

 bryophytes. 



Nevertheless, the thallophytes have developed some compli- 

 cated organs with highly differentiated tissues, as in the kelps, 

 rockweeds, red algse, sac fungi, and the higher basidia fungi, 

 such as the toadstools and mushrooms, puffballs, nest fungi, 

 and carrion fungi. These complexities are, however, very spe- 

 cial in character and not related to the structure of higher 

 groups of plants. 



