100 CHEADLE 



with them, together with an outer boundary, the pericycle. The patterns of 

 occurrence of vascular and associated tissues were to be reduced to a series of 

 evolutionary modifications that would show the continuity of changes and 

 something of how these changes were brought about. Some famous names 

 in botany (Van Tieghem, Brebner, Jeffrey) were involved in these investiga- 

 tions. Confusing terminologies developed because of unawareness of the com- 

 plexities of detailed differences in plant structure. Sometimes specious argu- 

 ments arose over boundaries, origin of various kinds of steles and therefore 

 their relative place in the evolutionary scheme devised for them, the influ- 

 ence of leaf traces, the interpretation of leaf bundles themselves, etc. It is no 

 wonder that those whose chief interests lay outside these arguments have, in 

 a word, cast a "plague on both your houses." They were unconsciously casting 

 a plague as well, perhaps, on scientists who overzealously categorize informa- 

 tion in the attempt to supply it to others in neat packages; organisms seem 

 to resist, so to speak, such compartmentalization. 



Those who were, or are, sympathetic to the evolutionary approach to 

 anatomy are likely to recollect the historical scenes upon which development 

 of the stelar concept proceeded. It should be recalled that the original notions 

 concerning the stele appeared not long after Darwin's Origin of Species had 

 motivated an immense surge of interest in evolutionary studies. Investigations 

 of the patterns of vascular tissue seemed to be natural developments of this 

 surge, and so these researches became quite fashionable for a time. If inter- 

 preted in this light, the stelar concept itself can be examined in its proper 

 context and thereby properly evaluated. It cannot be denied that this con- 

 cept has had an immense impact on anatomy and morphology. As Esau has 

 emphasized in briefly reviewing the concept in her book Plant Anatomy, 

 ". . . the stelar theory has been of unmistakable value in emphasizing the 

 unity of structure of the vascular system and in stimulating extensive com- 

 parative research." 



We know more about the details of conducting systems than we knew years 

 ago, and we know there are aspects of vascular-tissue patterns that do not 

 conform to the rigid terminologies employed in the stelar concept. We may 

 look forward some day to a scholarly review of this concept with the hope 

 that it may be used where instructive, and discarded where not, in our under- 

 standing of the vascular plant itself as a unit. 



Having arbitrarily disposed of the stelar concept as it refers to vascular 

 tissue, we may turn now to one aspect of xylem, the study of which was 

 influenced by the same motivation that resulted in the stelar concept. 



Xylem. In an earlier paragraph, the patterns of vascular tissues within 

 relatively young stems and roots were briefly discussed. If we turn our atten- 

 tion to the first xylem differentiated, in young stems particularly, we find that 

 the earliest-formed cells are commonly featured by annular thickenings (fig. 

 10) of secondary walls. These together with the later-formed cells with spiral 



