STUDY OF TKACIIEARY ELEMENTS IN PRIMARY XYLEM 87 



vascular rays, aiiel the sieve-tubes, vertical i)areiichyma, fibers 

 and traclieary elements. Usually procanibial cells are very much 

 alike in form and structure. (2) As seen in trans-section, pro- 

 canibial cells tend to be polygonal in form in contrast to the 

 rectangular or "box-like" shape of cambial cells. (3) The 

 maturation, especially of tracheary elements, from the cambium 

 is more abrupt than is the case in the procambium, and (4) the 

 radial walls of cambial cells are often noticeably thicker than 

 the tangential walls, a distinction which is not apparently char- 

 acteristic of procambial cells. In view of the above differences 

 and because of the exploratory state of the problem, the terms 

 "primary" and "secondary" will be utilized in this book as 

 convenient designations for the vascular tissues derived re- 

 spectively from the procambium and the cambium. But the 

 author is in full agreement with Esau's statement that procam- 

 bium and cambium are to be regarded "not as two distinct 

 meristems, but as two developmental stages of the vascular 

 meristem." 



II. Material for the Study of Tracheary Elements in Pri- 

 mary Xylem. — 



1. Transverse and lo7igi-sections of the hypocotyl of hean 

 seedlings. Treat the sections with phloroglucinol and hydro- 

 chloric acid,^ mount in water and examine under low and high 

 magnification. Note carefully the irregular arrangement of the 

 tracheary elements in the differentiating primary xylem as shown 

 in the trans-sections. The longi-sections, if cut in the radial plane 

 with respect to the primary xylem, will show the order of appear- 

 ance and the types of secondary wall-patterns in the tracheary 

 elements of the protoxylem and metaxylem. 



2. Prepared and stained transverse and longi-sections of the 

 stem of Trifolinm,. Examine the trans-sections noting partic- 

 ularly the regular arrangement of the tracheary elements in the 

 primary xylem of the collateral vascular bundles. A study of 

 the longi-sections will reveal not only the various types of wall- 

 patterns in the successive tracheary elements, but will also show 

 the effects of stretching on the primary and secondary walls of 

 the annular and spiral elements of the protoxylem. 



1 Cf. Appendix, p. 141. 



