SOLANUM TUBEROSUM 541 



of the small scale leaves which occur in the same phyllotaxy as 

 the foliage leaves. Each "eye" may contain three or more buds. 

 (Fig. 2.87.) As the tuber enlarges, new buds are formed at its 

 apex, and the older ones develop to maturity, establishing vascular 

 connections with the stele. 



Ontogeny of the Leaf. — The development of the leaf has been 

 described by de Vries (i6) and Artschwager (i). 



The leaf primordia arise from the growing point of the stem 

 as small protuberances which elongate and curve adaxially so that 

 they finally arch over the growing point. While this growth and 

 curvature are taking place, the primordia undergo form changes, 

 the first being the production of lateral projections on both sides 

 of the primordium that later become the blades of the terminal 

 leaflets. Very early in ontogeny, the terminal leaflet consists of 

 a short stalk and blade with a well-developed midrib, and the 

 halves of the lamina are folded so that their adaxial faces oppose 

 each other. For some time, the terminal leaflet is far advanced 

 in its development over the other leaflets which diff"erentiate 

 gradually, and the slow elongation of the axis of the leaf provides 

 space for the formation of the primordia of the lateral leaflets. 

 Artschwager (i) states that "the diff"erence in the rate of develop- 

 ment is so great that the terminal leaflet is already one centimeter 

 in length and has become green long before the rest of the leaflets 

 appear." The development of the lateral leaflets occurs in basip- 

 etal succession and the uppermost leaflets are green and well 

 developed when the lower ones are still primordia. 



The first epidermal hairs appear early on the abaxial surface 

 of the primordium, developing acropetally; and, later, hairs are 

 produced on the adaxial surface in the same order. These arise 

 in longitudinal rows adjacent to the veins, and the first glandular 

 ones are soon followed by hairs that are stiff" and unicellular. 

 Both kinds increase in number rapidly and mature before the 

 mesophyll is completely differentiated. While the leaf is small, 

 the hairs form a dense mat over the veins; but, since no new hairs 

 are formed, the density decreases as the leaf enlarges and they are 

 widely scattered at maturity. 



In the development of the mesophyll, the cells adjacent to the 

 upper epidermis elongate to twice their original height and form 

 a compact palisade region; while the remaining parenchymatous 

 cells comprising the spongy region are approximately isodiametric. 



