32 PLANTS AND MAN 



into which it has passed from the rhizoids. Carbon dioxide and 

 oxygen diffuse into the plant over the entire epidermis or through 

 special pores in the surface. 



For successful living in a greater variety of land habitats than 

 that found in Marchantia, further structural specialization is 

 necessary, with a more efficient functioning of the absorptive and 

 photosynthetic tissues. Roots take the place of rhizoids and the 



///? POR£. 



~ EPIDERMIS 



PHOTOSYNTHETIC 

 CELLS 



CHLOROPLASTS 



STORAGE CELLS 



RHIZO/DS 



Fig. 12.— Division of labor in Marchantia results in a protective epidermis, 

 photosynthetic cells, storage cells and absorptive rhizoids. 



photosynthetic tissues are collected in expansions of the body 

 known as leaves. Skeletal and conductive tissues are important in 

 the stems, which by necessity connect the roots with the leaves as 

 well as support the latter. The leaf becomes the most character- 

 istic plant organ; in fact many may consider leaves the identify- 

 ing feature of a plant. Each leaf is an organ made up of tissues 

 specialized in carrying on photosynthesis; it is the last word in the 

 evolution of that part of the green plant concerned with construc- 

 tive metabolism. Thallus plants and liverworts have no leaves, 



