THE PLANT-BODY 



17 



outer cells in which the greater part of the chloroplasts are placed, 

 and it is clear that a provision for the most favorable exposure of 

 the green cells to light is one of the principal causes for many of 

 these modifications of the plant-body. 



The Thallus 



The increase of the area of green tissue is attained in another way 

 in many of the lower plants, where the plant-body has the form of 

 a flat plate or Thallus. 

 A simple example of 

 this is the common 

 Sea-lettuce (Ulva), and 

 larger examples are 

 many of the Kelps, or 

 Brown Algae (Fig. 7). 

 This type of plant-body 

 is the result of cell- 

 division in two planes, 

 so as to form a single 

 layer of cells, which in 

 most cases later be- 

 comes thicker by divi- 

 sions in a third plane 

 also. A thallus of much 

 the same structure is 

 found in the lower 

 Mosses or Liverworts, 

 and in the sexual plants 

 (Gametophyte) of many 

 Ferns (Fig. 7, C). 



A somewhat different 

 type of thallose body 

 is seen in the peculiar 

 plants known as Fungi, 

 which differ from the 

 Algae in not possessing 

 chlorophyll. In these the plant-body is made up of filaments (Hy- 

 phae) which may form a loose, fluffy mass as in the common Moulds, 

 or may be closely interwoven into a thallus of definite form as in 

 many Lichens. Most of them produce characteristic fruiting struc- 

 tures (Sporophores) which are composed of densely interwoven and 

 frequently coherent hyphae, so that in section they often present the 

 appearance of a true tissue like those of the higher plants (Fig. 8), 

 although these masses of tissue are the result of the coalescence of 



FIG. 7. A, Thallus of Ulva lactucu, slightly reduced ; 

 B, young plant of Laminaria Farlowii, showing 

 the stem and holdfast or " root," r, slightly re- 

 duced ; C, prothallium of a Fern (Struthiopteris 

 Germanica) ; r, root-hairs (X 8). 



