SOLANUM TUBEROSUM 535 



while the parenchymatous tissues of pith and cortex comprise 

 nearly two-thirds of the total area. 



Crafts (9) has investigated the structure of the phloem in relation 

 to the problem of translocation. He rejects theories to account 

 for the rapid transport of carbohydrates to the tuber that are based 

 upon diffusion, protoplasmic streaming, or movement through 

 young sieve tubes, conducting parenchyma, and perforations in 



Fig. 184. Basal portion of plant showing habit of mature tubers. (Photograph by J. 



Horace McFarland Co.) 



sieve plates. Instead, he proposes that "Diffusion along plas- 

 modesma of cross walls and acceleration by protoplasmic streaming 

 within non-vascular tissues, combined with pressure flow through 

 permeable sieve tubes and phloem walls within specialized con- 

 ducting organs, seem most satisfactorily to explain translocation 

 in the potato." 



Anatomy of the Mature Tuber. — The mature tuber is mor- 

 phologically a short thickened stem bearing scale leaves that fall 

 off early in ontogeny, leaving prominent scars. (Fig. 184.) In 



