Lecture XXII. 



189 



the protophloem is also formed on its surface, at places alternating 

 with the groups of protoxylem. The primary wood differentiates 

 in the central part of the procambium and forms a core of wood 

 connecting the protoxylem threads. The protoxylem groups thus 

 lie on the outside of the primary wood. The woody core of the 

 root is consequently described as exarch. In the pine there are 

 usually three groups of protoxylem, so that the primary- wood-core 

 is a three -sided prism with protoxylem threads upon its angles. 

 On the flat, or slightly grooved, sides of the prism are the threads 



P Cx C Xy En 



Re 

 P 



FIG. 57. Pinus silvestris, transverse section of root, x 40. P, piliferous 

 layer ; Cx, cortex ; En, endodermis ; P, phloem ; C, cambium ; Xy, 

 xylem ; Re, resin-canal. (Photomicrograph by Flatters, Milbourne, & 

 McKechnie.) (From Evans' An Intermediate Textbook of Botany.) 



of protophloem. Between them and the wood is the cambium. 

 While the primary wood is differentiating the cambium forms 

 primary bast on the inside of the protophloem and, by the formation 

 of tracheids on the flat face of the woody prism, converts it into a 

 cylindrical core of wood. Interfascicular cambium now forms out- 

 side the protoxylem threads, and unites the cambium into a con- 

 tinuous sleeve ensheathing the;core of primary wood and protoxylem. 

 The formation of secondary wood by this continuous cambium 

 increases the thickness of the woody cylinder, just in the same 



