SOLANUM TUBEROSUM 537 



the axil of each leaf is a compound bud consisting of an unelongated 

 branch bearing several rudimentary leaves and axillary buds. The 

 central bud, which is the most prominent, is the first to develop 

 when growth is resumed. The arrangement of the leaves is spiral 

 and agrees with the phyllotaxy of the vegetative stem. 



Tuberization results from a cessation of the polar growth of the 

 axis of the rhizome and a marked lateral proliferation of storage 

 tissues which form the main bulk of the mature tuber. The 

 anatomy of the tuber has been variously interpreted. De Vries 

 (16), in his classic work on the potato, regarded the tissue of the 

 tuber as being chiefly vascular and derived from the cambium, 

 differing from the normal cambial product only in that the paren- 

 chyma produced was thin-walled storage tissue rather than woody 

 parenchyma. Esmarch-Bromberg (ii) attributed the principal 

 growth of the tuber to an enlargement of the medullary region, 

 and regarded the inner phloem groups as having an independent 

 origin unrelated to the vascular cylinder. Reed (18) ascribed 

 the growth of the tuber to the activity of three parenchymatous 

 regions, the medulla, phloem, and cortex; and Artschwager (x) 

 is in essential agreement with Reed except with reference to 

 the activity of the inner phloem parenchyma. In regard to this 

 region. Reed states that the cells between the inner phloem and 

 the protoxylem begin to divide very early in ontogeny, causing a 

 spreading out or dispersion of the phloem groups. He also holds 

 that the parenchyma of the inner phloem groups is very active, 

 contributing a considerable amount of tissue to the growing tuber. 

 Artschwager expresses doubt on this point, and attributes the 

 increased amount of storage parenchyma to the proliferation of 

 cells in the procambial zone, especially in the perimedullary re- 

 gion, rather than to an activity of the parenchyma of the inner 

 phloem. 



The principal zones in the mature tuber from the periphery 

 inward are the periderm, cortex, vascular cylinder, perimedullary 

 zone, and central pith. (Fig. 185, A.^ The periderm is six to 

 ten cell layers in thickness and acts as a protective zone over the 

 entire tuber, being broken only by small lenticel-like structures. 

 These develop in the tissue underlying the stomata and are initiated 

 when the young tuber still retains its epidermis. There is con- 

 siderable variation in the width and character of the periderm in 

 different varieties of potatoes, some of which are sufficiently 



