86 TRACIIEARY ELEMENTS 



careful eytologieal .study of this problem from an extensive as 

 Avell as an intensive standpoint is needed in advance of any 

 generalizations. With the aid of recent improvements in plant 

 microtechnique, it should be possible to investigate successfully 

 the precise relationships between the cytoplasm and the develop- 

 ment of specific wall-patterns in tracheary elements. 



3. The distinction between primary and secondary vascular 

 tissues. In practice, great difficulty is experienced in attempting 

 to distinguish the boundaries between the primary and secondary 

 vascular tissues especially in leaves and young stems. Protoxy- 

 lem, because of the definitive characters of the secondary wall 

 patterns, is usually readily demarcated but the limits between 

 metaphloem and secondary phloem on the one hand, and meta- 

 xylem and secondary xylem on the other hand, are often difficult 

 or indeed impossible to draw on the basis of adult structure. One 

 of the criteria often employed is based on the idea that the con- 

 ducting elements of secondary vascular tissues, since they orig- 

 inate from periclinal derivatives of the cambium, are arranged 

 in more or less regular radial rows in contrast to their irregular 

 arrangement in primary phloem and primary xylem. In a recent 

 review of this whole problem, Esau (1938, pp. 356-361) has shown, 

 however, that according to many ontogenetic studies, the first- 

 formed or "primary" vascular tissues "may be arranged in an 

 orderly manner." A good illustration is furnished by the radial 

 alignment of both "primary" and "secondary" tracheary ele- 

 ments in the vascular bundle of Trifolium (Eames and Mac- 

 Daniels, p. 255, Fig. 117A). Evidently then, the method of ar- 

 rangement of tracheary cells does not provide a consistent basis 

 for demarcating pi-imary and secondary xylem. However, when 

 certain differences are analysed between the so-called procam- 

 bium, wliich produces the primary vascular tissue and the "cam- 

 bium" which forms secondary vascular tissues, it appears tiiat 

 the distinction between the two tissues may have some justifica- 

 tion ontogenetically. 



According to Esau, four significant differences can be recog- 

 nized, viz.: (1) in organs with well-defined secondary growth, 

 two kinds of initials, the /v/// and fusiform types, are character- 

 istic of the cambium. These initials produce respectively the 



