SOLANUM TUBEROSUM 



515 



in transection. In some cases, the angular margins form ridges or 

 wings which are prominent in young plants. (Fig. xyi.) 



The Leaf. — The leaves are alternate with a ^f 3 phyllotaxy, 

 and the spiral is usually counter-clockwise. The petioles are 

 semicircular in transection with a convex abaxial and a slightly 

 concave adaxial surface. The base 

 of the petiole is flattened some- 

 what and ensheathes about one- 

 third of the circumference of the 

 stem at the node. The wing-like 

 margins of the petiole are une- 

 qually decurrent, one extending 

 down the stem through a single 

 internode, the other through two. 

 In plants grown from seed, the 

 first leaves above the cotyledons 

 are simple, and this may be the 

 case with the first leaves arising 

 on stems grown from tubers. (Fig. 

 zyx.) The later leaves are com- 

 pound and irregularly odd-pinnate 

 with more or less petioled leaflets. 



The number of leaflets varies 

 considerably with the variety, 

 and, in addition to the terminal 

 one, there are usually three or four 

 pairs of large, oval leaflets with en- 

 tire or serrate margins. Smaller, 

 secondary leaflets sometimes occur between the large primary 

 ones. The young leaflets are densely pubescent, bearing hairs 

 of several types, some of which are long ^nd straight consisting 

 of one to several cells, while others are short and glandular 

 with a spherical head of four cells which is borne upon a short 

 slender stalk cell. At maturity, the leaf is sparingly pubescent, 

 the persistent hairs occurring chiefly along the midrib and lateral 

 veins. 



The leaflets are net-veined; and branches arise from the promi- 

 nent midrib, forming a dense reticulate system. The lateral veins 

 extend toward the apex and margins of the leaf, anastomosing 

 freely so that the marginal portions have a greater compactness 



Fig. 171. Basal portion of potato plant 

 showing development of rhizomes and 

 young tubers. 



