ixL THE STRUCTURE OF ECONOMIC PLANTS 



of the primary root. The seminal roots may be somewhat smaller 

 than the primary root with fewer protoxylem points. The princi- 

 pal adventitious or prop roots are much larger than the primary 

 root, and the number of xylem points may be double that found in 

 the primary stele. Another difference exists in the aerial portions 

 of the adventitious roots where the epidermis instead of disinte- 

 grating is persistent, developing a cuticle similar to that of the 

 stem. 



Adventitious roots may arise in the pericyclic region of the first 

 internode and in the meristematic basal portions of any of the 

 higher internodes. A root primordium is differentiated from a 

 ring of meristematic tissue which is comparable in function and 

 location to a pericyclic zone although there is no definite endoder- 

 mis to determine its outer limit. The elongating root then forces 

 its way through the cortex and the basal portion of the leaf which 

 ensheathes the node. The emergence of the lateral is accomplished 

 by a splitting of the cells of the regions penetrated and by digestive 

 action. (Fig. xi.) 



Hypocotyl and Stem. — The hypocotyl is very limited in extent 

 and strictly root-like in structure so that the description given 

 for the mature primary root applies to the axis up to the cotyle- 

 donary node. The vascular transition is epicotyledonary, occur- 

 ring chiefly in the vascular plate of the first node and in the first 

 internode. It is continued to some degree in the second internode 

 in which transition bundles may be found; and, occasionally, there 

 may be some transitional structures in the third and fourth inter- 

 nodes. 



The First Internode. — The first internode has a central pith 

 with an annular zone of vascular elements bounded by a pericycle 

 and endodermis, both of which are one cell layer in thickness. 

 (Fig. 48.) The cortex, consisting of thin-walled parenchyma, is 

 several cells in thickness and the epidermal cells have a thin cuticle. 

 In the stelar region, there are groups of exarch xylem which usually 

 alternate with endarch collateral bundles. The primary phloem 

 is in a collateral position with respect to the endarch primary 

 xylem; and in an alternate, radial position in relation to the exarch 

 xylem groups. Thus, the transitional region exhibits an intermix- 

 ture of root-like and stem-like orientations of the vascular tissues. 



The Coleoptile. — The coleoptile is the second leaf of the seed- 

 ling axis and arises at the second node. Its elevation to approxi- 



