THE ANGIOSPERMAE : STEMS 



857 



Each procambial strand originates in very close proximity to the base of a 

 leaf primordium or actually in contact with its cells, and the strand in the 

 stem is continuous with that which develops in the primordium itself. Every 

 vascular bundle of the stem is thus a part of the vascular supply to a leaf, or, 

 in other words, is a leaf trace, and is so from the procambial stage onwards. 

 In other words, the vasculation in the stem does not belong to the axis per se, 

 but to the leaves. Abnormal exceptions do not invalidate this general rule. 



Fig. 843. — Diagram of the course of vascular diflferentia- 

 tion at the apex of a stem of Nicotiana. Procambium 

 stippled ; xylem black and white ; phloem black. 

 The arrows indicate the direction in which differ- 

 entiation is taking place. {After Esau.) 



Although procambial connection between stem and leaf exists from the 

 very beginning of the leaf, the development of vascular cells in the pro- 

 cambium does not begin until three or four plastochrons later (Fig. 843). 

 Phloem development begins first. The first sieve tube makes its appearance 

 at the outer edge of the procambial strand and is from the first connected in 

 linear sequence with those in the older part below it. The differentiation of 

 this sequence of sieve tube units progresses slowly upwards into the leaf, 

 but it does not, as a rule, reach quite to the apex of the leaf, where sieve 

 tubes may be absent even at maturity. 



The first xylem element, normally a spiral tracheid, appears at the inner 

 edge of the procambium. It lies close to the base of the leaf primordium into 



28 A 



