RESEARCH ON XYLEM AND PHLOEM 

 Progress in Fifty Years 



Vernon I. Cheadle 



To many students in elementary botany classes who must learn something 

 about structure of plants, even if only to practice use of the microscope, vas- 

 cular tissues do not often excite much interest. And if one reads elementary 

 textbooks, he recognizes that most of these do not serve very well in support- 

 ing the teaching efforts of instructors, who, unhappily, are seldom very con- 

 fident of their own understanding of these tissues. 



Yet a knowledge of vascular tissues is immensely important to an under- 

 standing not only of the activities of vascular plants in their present environ- 

 ment, but also of how they must have survived and prospered through the 

 ages. Furthermore, these tissues or products derived from them have been 

 important items in Man's economy since the time of his arrival as a thinking 

 organism. This article is a review of progress in research during the last fifty 

 years on some specific structural aspects of these important tissues. 



Introduction. Vascular tissues are chiefly involved in rapid conduction 

 in plants. They are composed of xylem — the chief water-conducting and sup- 

 porting tissue — and of phloem — the chief food-conducting tissue. These seem 

 to be prosaic definitions, but notice that though they mention nothing about 

 structural details, they do provide the clues to the ability of plants to live on 

 land. Is this economically or biologically important? Our economy is based 

 first and foremost on plants — plants that grow on land — and they succeed 

 in a land environment mainly (although of course not wholly) because they 

 have xylem and phloem. 



There are fascinating controversies among botanists concerning the origin 

 of land plants. These controversies revolve around subject matter, most of 

 which need not concern us here; but whatever stand one may take, he must 

 agree that without xylem and phloem land plants could never have evolved 

 to their present state of development. It seems generally agreed that land 



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