R. N. Salaman and J. W. Lesley 23 



perhaps loss of turgidity which causes the stems to become spreading 

 or even to lie on the ground. 



Before proceeding to an analysis of the anatomy of the variety it 

 will be well to give an outline of the structure of an adult aerial potato 

 stem. With so many distinct varieties of potato it is perhaps not sur- 

 prising that we find somewhat divergent accounts of its anatomy and 

 development. 



The following account is, as far as we know, true of all cultivated 

 varieties, and we shall therefore venture to call it the normal type. 



The anatomical structure of the aerial stem of the potato normally 

 resembles that of an ordinary herbaceous dicotyledon with the peculi- 

 arity common to nearly all. Solanaceae of medullary phloem (Plate IV, 

 fig. 1). The adult stem is roughly triangular in the intern ode and 

 furnished with wings at the corners. A single outer layer sometimes 

 beset with a few hairs constitutes the epidermis. The cortex is differ- 

 entiated into narrow collenchymatous cells followed by a large celled 

 inner zone with thin walls, while an innermost layer of slightly smaller 

 cells ends in an endodermis often containing starch grains. The stele 

 consists of a number of bundles united by secondary growth so as to 

 form a continuous ring of phloem and xylera separated by the cambium 

 and encircling a large pith containing islands of medullary phloem. 

 The outer and inner phloem consists of parenchyma, fibres, sieve tubes 

 and companion cells. As was noted above, the stem is for the most 

 part roughly triangular, the sides of the triangle being flat or slightly 

 convex. This shape is due to the presence of three main vascular 

 bundles between which are much smaller bundles. The strands of 

 fascicular cambium become connected by the interfascicular cambium 

 arising in the medullary rays, and thus completed this tissue continues 

 to form phloem outwards and xylem inwards. As a result an almost 

 complete cylinder of vascular tissue is quickly formed. The -protoxylem 

 is readily recognized bordering the pith and consists of narrow elements. 

 In the region of the actual bundles the secondary xylem, in addition to 

 numerous narrower elements, contains large vessels especially in the three 

 main bundles. The secondary xylem elements are thick-walled and are 

 arranged in radial rows corresponding to their origin fi-om the same 

 meristematic cell. The interfascicular xylem consists almost exclusively 

 of narrow thick-walled elements. Both the fascicular and the inter- 

 fascicular xylem are lignified. The ground tissue of the pith is similar 

 to that of the inner cortex. It contains no thick-walled elements with 

 mechanical functions. The outer pith zone adjoining the xylem consists 



