THE EVOLUTION OF THE PLANT BODY 29 



rials after a primitive fashion. Attachment to the substratum is 

 the special duty of the holdfast cells, which however have no 

 absorptive functions. The great size of these kelps — they are the 

 largest thallus plants — is due in part to this beginning of tissue 

 differentiation. The life of a kelp is basically the same as that of 



Fig. 10. — The body of a kelp is a thallus with division into a root-like holdfast, 

 a flexible slender stalk, and an expanded (often subdivided) blade. 



Spirogyra, except for the mutual inter-dependence of the thallus 

 cells. Carbon dioxide, water and salts diffuse in through the 

 epidermal layers and pass eventually to the photosynthetic cells. 

 After food manufacture the excess glucose passes out from the 

 green cells to all parts of the plant, to be used as needed. Since 



