28 PLANTS AND MAN 



medium with which every cell is in contact, and waste products 

 pass outward in the same way. When excess protoplasm has 

 been formed within a cell, it develops a transverse wall and cell 

 division results in two cells where formerly there had been one. 

 Growth of the plant as a whole is the result of this increased 

 number of cells. Some materials in solution diffuse from one 

 cell to its neighbor but no cell is dependent upon this means of 

 support. No special protective walls are developed; when a cell 

 or group of cells has been injured and dies, the fragments form 

 new filaments by the customary cell division. In its life activities, 

 even though it is multicellular and macroscopic in size, Spirogyra 

 does not differ essentially from the microscopic and unicellular 

 desmid. 



A Multicellular Thallus Plant with Some Division of Labor 



In those aquatic plants known as kelps (fig. 10) the multi- 

 cellular body shows an increase in complexity with a beginning of 

 division of labor. Kelps are brownish-green seaweeds which live 

 in cool oceans, often forming meadows along the sea bottom be- 

 low low tide mark. The body of a typical kelp, such as Laminaria, 

 is made up of millions of cells forming a massive thallus which 

 sometimes attains a length of a hundred feet or more. There 

 is a definite division of the body into three parts; a root-like basal 

 holdfast, a slender stem or stalk, and an expanded blade. Being 

 aquatic, the kelps like the desmids and pond scums can absorb all 

 of the necessary substances for autotrophic metabolism from the 

 surrounding medium. There are two important diflferences, 

 however. Only a small proportion of the cells of the kelp's body 

 is in contact with the water, so that these must form a medium of 

 exchange of substances between the water and the remaining 

 cells of the thallus. Furthermore, all of the cells do not contain 

 chloroplasts; such cells are located for the most' part in the blade 

 where photosynthetic activity is centralized. The restriction of 

 photosynthetic tissue to certain parts of the plant body becomes 

 more and more marked as we proceed to the higher plants, so in 

 this respect the kelps introduce a new element in the construc- 

 tion of the plant body. The stem is made up of outer protective 

 cells and an inner strand of elongated cells which conduct mate- 



