106 



BOTANY 



concentric bundles in such stems would be homologous with the single central 

 concentric or radial bundle. According to this assumption, which is the essential 

 of the STKLAR THEORY ( 60 ), the single central bundle is termed the stele, and the 

 circle of collateral or concentric bundles with the enclosed pith would also be 

 regarded as a stele since it is derived from the primitive stele. A single bundle 

 may therefore represent the whole stele or a part of the stele. There is usually 

 only one stele or central cylinder in the stem of the Spermatophyta (monostely). 

 Cases are, however, met with when the stele is divided (polystely) as in the stems 

 of Auricula or Gunnera. 



(y) The Leaves. 1. Development of the Leaves. The leaves 



Fio. 122. Acer platanoides. A, External view of a bud, with two young leaves between which the 

 apical cone of the stem is visible ; sp, the leaf-blade, in which live segments are indicated, 

 the uppermost one being developed first ; st, the zone, by the growth of which the leaf-stalk 

 will arise later. B, An older leaf seen from the side ; the young vascular bundles, which will 

 later determine the venation, are indicated. (7, Fully-grown leaf, with the course of the 

 vascular bundles indicated diagrammatically. D, A transverse section of the basal portion of 

 a bud showing three vascular bundles in each leaf. E, A similar section at a higher level ; 

 the number of vascular bundles has increased by branching. (After DEINEGA, from GOEBEL'S 

 Organography. A, B, and E slightly magnified.) 



have been seen to arise exogenously at the growing point of the stem 

 as lateral papillae or bulges (Fig. 98, 102/), which to begin with are 

 unsegmented. These are the LEAF PRIMORDIA (Fig ; 125 A, b). 

 Usually a young leaf occupies only a part of the circumference of the 

 apex, but it may encircle the latter as an annular ridge. Several 



