IPOMOEA BATATUS 509 



large vessels have bordered pits and are surrounded by smaller ones 

 and by parenchyma that becomes thick-walled as the stem matures. 

 The pits of the wood parenchyma are simple, and half-bordered 

 ones occur where the vessels and parenchyma are adjacent to each 

 other. The interfascicular regions are characterized by radial 

 series of thick-walled elements, and groups two to five rows in 

 width are separated from adjoining ones by a row of thin-walled 

 ray-parenchyma cells. 



Between the protoxylem and the inner phloem, there is a region 

 of thin-walled parenchyma; and, in this zone, an inner cambium 

 may arise which is relatively inactive and seldom forms a continu- 

 ous inner cambial ring. It usually produces additional phloem 

 centripetally; and, less frequently, may cut off a few secondary 

 xylem elements centrifugally, in w^hich case bundles with a reverse 

 orientation are formed. The inner phloem consists of scattered 

 groups of companion cells and sieve tubes, and the pith is as 

 described for the young stem. 



The Development of the Foliage Leaves. — At the time of the 

 differentiation of the first foliage leaves, the epicotyledonary 

 axis is surrounded by the bases of the cotyledons which form a 

 partial sheath or cotyledonary collar. In the ontogeny of the 

 leaf, its primordium arises from the peripheral tissues of the grow- 

 ing point as a conical mass of cells. The primordium elongates 

 and broadens by general meristematic activity until it becomes 

 somewhat crescentic or triangular in transection at the base, 

 tapering to a rounded or slightly subtriangular apex. (Fig. 2.68, 

 /4.) In the basal portion of the primordium which later forms the 

 petiole of the leaf there is an early differentiation of procambial 

 strands. Growth in length of the primordium is accomplished 

 by general cell division, but there is an acceleration of meristematic 

 activity in tiie lateral portions of the adaxial surface. (Fig. 

 i68, B.) This results in the formation of the lamina which 

 develops with the upper surfaces of the two halves of the blade 

 opposing each other. They are thus oriented at right angles to 

 the midrib with their edges directed toward the axis of the epicotyl. 

 (Fig. x68, C.) Following differentiation of the lamina, the petiole 

 elongates by continued cell division and enlargement. 



Glandular scales are formed in the epidermal tissue of the 

 developing leaf. These are more numerous on the exposed lower 

 epidermis than on the upper and resemble the scales found on the 



