CHAPTER V. 



ANATOMY AND HISTOLOGY. 



While the anatomy of the Compositas has been studied in much 

 detail, beginning with von Sachs, followed by van Tieghem, Vesque, 

 Vuillemin, Col, and less voluminously by other writers, that of the genus 

 Parthenium had, up to 1901, received no examination. In that year the 

 plant which supplies the object of the present treatise came to the atten- 

 tion of the French botanists MM. Fron et Francois (1901), who gave a 

 brief account of the more obvious features of the anatomy of the stem 

 and of the structure of the fruit. A more extensive paper was published 

 in 1908 by Dr. H. Ross, who visited Mexico in 1907 and examined the 

 guayule in the field, chiefly about Saltillo. In this paper an anatomical 

 study of guayule was supplemented by brief reference to two other species, 

 P. incanum and P. tomentosum. To both these contributions, as also to 

 those of more general import, reference will presently be made. 



ROOT. 

 PRIMARY STRUCTURE. 



The primary root is diarch. The two bundles of protohadrome, of 

 spiral vessels, become early united by a centripetal development of vessels 

 forming a primary plate, on either side of which stand the two protolep- 

 tome strands. At this time the stele has a continuous pericambium and 

 is surrounded by a well-marked endodermis, which may be recognized by 

 the bands of Caspary and by the starch-content of the cells (plate 22, figs. 

 6-8). The starch-grains are relatively large and are compound. Their 

 persistence is variable, traces being visible for some months in some 

 instances, e.g., in a root 4 mm. in diameter; in other cases they may have 

 disappeared in a few weeks. Thus in a root 0.46 mm. in diameter, in 

 which radial thickening of the endodermis had just commenced, starch in 

 these cells fluctuates, there being now more and now less, apparently 

 according to the draft upon it by the tissues. Without the endodermis 

 lie three layers of cortical cells with extensive intercellular spaces, which, 

 however, do not occur between the outer layer of cortical cells (the hypo- 

 dermis) and the epidermis. 



SECONDARY STRUCTURE. 



The epidermis begins very early to break down, so that in a root less 

 than 0.5 mm. in diameter the earliest peridermal divisions have set in. 

 These do not usually occur in the outermost cortical cells, which here 

 take on, in a weak fashion, the characters of an exodermis, as described 

 for Cephalanthus and Tecoma by Holm (1907), but in the second hypo- 

 dermal layer (plate 22, fig. 7). At this time growth commences in the 

 cortex, both radial and periclinal divisions occurring (plate 22, fig. 8). 

 Growth of the endodermis is concurrent (plate 2 2 , fig. 6) . Both radial and 

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