STRUCTURE OF CONDUCTING ELEMENT 305 



any considerable part in connection with intercellular translocation. 

 It is nevertheless quite possible that they acquire greater importance 

 as paths of conduction in special cases, for instance, in certain endo- 

 sperms. Wherever, in particular, the canals are unusually wide they 

 may have this significance ; in the case of many sieve-tubes, for 

 example, it is certain that large quantities of protein material pass 

 through the comparatively wide pores of the sieve-plates. 



The reduction of the transverse partitions in the conducting tubes 

 is carried a step further, where the limiting membranes sooner or later 

 become completely obliterated. This condition is exemplified by the 

 perforated transverse walls of the latex-tubes of Mum and Chelidonium. 

 A far more important illustration is provided by the tracheae or true 

 vessels which, among Angiosperms, constitute the principal conducting 

 elements, both in the primary hadrome and in the secondary wood. 

 Here the transverse walls are sometimes only perforated by several large 

 openings ; more often, however, they disappear altogether except for a 

 narrow marginal ring, which persists and marks the former position of 

 the septum. A still more complete reduction of the transverse walls 

 occurs in the latex-vessels of the majority of Cichoriaceae, Papayera- 

 ceae, Papayaceae, etc., which in the adult condition show no trace 

 whatever of septation. 



The final stage in this progressive degeneration of transverse walls 

 is exemplified by coenocytic latex-cells of the type found in the genus 

 Euphorbia, which are devoid of septa from their first formation. These 

 structures do not arise by the fusion of rows of cells ; each, on the 

 contrary, consists of a single cell that has become enormously elongated 

 and abundantly ramified, so as to give rise to a complex system of 

 tubes, which push their way among the other tissues like the hyphae 

 of a parasitic Fungus. 



In addition to their elongated form, and the pitted or perforated 

 condition of the transverse walls, conducting elements often display yet 

 another histological feature which tends to facilitate the passage of 

 material, namely, an increase in the surface available for diffusion. 

 This increase is effected by the enlargement of the area, either of the 

 entire transverse wall or of the limiting membrane of the pits borne 

 thereon. The total area of a septum may be augmented in two 

 different ways. In the one case the transverse wall retains its primary 

 orientation, but is enabled to expand by the dilatation of the cells 

 on both sides of the septum ; this condition is exemplified by the 

 sieve-tubes of primary leptome strands, by the peculiar latex-cells 

 which occur in the bulb-scales of Allium, and by the conducting 

 elements in the stem of Polytrichum (both in the central strand and in 

 the leaf-traces). Inci eased exposure of surface is attained in a different 



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