356 THE STRUCTURE OF ECONOMIC PLANTS 



tissue tension, since the epidermal cells are under tension and the 

 pith cells under pressure; but, when placed in auxin solution, there 

 is an inward curvature of the two halves which can be measured to 

 determine the auxin concentration. 



The Fourth Internode. — In the fourth internode, a definite 

 stem pattern is laid down, and the transitional situation existing 

 in the lower internodes, in which both exarch and endarch primary 

 xylem groups are found, no longer occurs. The stem is subquad- 

 rangular and becomes definitely four-sided at internodes above. 

 The endodermis is not as well defined as in lower internodes and 

 in the stele there are two polar bundles which supply the leaves, 

 and, at right angles to them, two bundles which supply the 

 stipules. (Fig. 178 and Fig. 179, D.) Higher in the stem the 

 number of vascular bundles in the ring is increased because of the 

 downward divergence of vascular traces from the vegetative and 

 floral axes which develop in the axils of the upper foliage 

 leaves. 



Gourley (11) in his study of the course of the cortical fibro- 

 vascular bundles showed that they supply the lateral lobes of the 

 trifid bracts, which may be interpreted as homologous to stipules, 

 and the stipules of the vegetative leaves. The polar bundles of the 

 stele provide the chief vascular supply for the central lobe of the 

 trifid bract, and for the lamina of the compound leaf. The cortical 

 fiber bundles which are in the same vertical plane as the polar 

 bundles become a part of the mechanical tissue of the latter when 

 they diverge outward in the nodal region. 



The course of the cortical fibrovascular bundles which are located 

 at approximate right angles to the plane of phyllotaxy is as follows : 

 beginning at the cotyledonary node, they diverge from the margins 

 of the vascular bundles which supply the cotyledons. In some 

 instances, two may arise on each side, but only one of these persists. 

 They extend upward in the cortex to the second node, where a 

 branch from each supplies the lateral lobes of the first trifid bract. 

 Within the bract these bundles anastomose with the median stelar 

 bundle so that the bract appears to have a single vascular strand. 

 The main portion of each cortical vascular bundle is continued up 

 through the second internode; and, at the third node, each bundle 

 again branches to supply lateral veins to the lobes of the second 

 bract. The main cortical vascular bundles continue through the 

 third internode; and, at the fourth node, branches from each follow 



