THE PLANT BODY 



63 



bringing about the most favorable exposure to light of the 

 photosynthetic apparatus. 



Another method of attaining the same object is found in 

 other Seaweeds. In the common Sea Lettuce (Ulva) and 

 the Rockweed (Fucus) the plant body takes the form of a 

 plate of cells, as a result of cell division occurring in two 

 planes, and then this THALLUS usually becomes thicker by 

 division of the cells in a third plane also. As a result of 

 further modifications of the thallus, the single attaching cell 

 of the simple filamentous types is replaced in the larger Sea- 



FIG. 24. The Giant Kelp. A marine Alga 

 which may attain more than 200 feet in length. 

 A thallus plant exhibiting distinct leaf-like and 

 stem-like structures, and holdfast. (From 

 Ganong.) 



weeds by massive HOLDFASTS which anchor them securely 

 to rocks. Still there is no marked differentiation in the 

 cellular components of the holdfasts because they perform 

 only this one function of the roots of higher forms; the ab- 

 sorption of food materials dissolved in the water being 

 carried on by the individual cells of the whole plant. Al- 

 though among the most complex Seaweeds, for example in 

 the Kelps and Gulf weed, the form of the thallus is highly 

 modified into divisions which serve certain of the func- 

 tions of root, stem, and leaf of higher plants, still none of 

 the fundamental tissue differentiations so characteristic of 

 the higher forms occur. Similarity of function has given 

 rise to ANALOGOUS structures. (Figs. 23, 24, 25.) 



