366 CONDUCTING SYSTEM 



or more rows of spiral or closely reticulate tracheides. Each of these 

 small water-conducting strands is enclosed in a carbohydrate-transport- 

 ing parenchymatous sheath, which generally extends right to the end of 

 the tracheide-bundle (Fig. 146), forming a sort of cap or hood over its 

 slightly dilated extremity. In exceptional cases the distal ends of the 

 [terminal] tracheides- may project freely into an intercellular space ; 

 this condition occurs very frequently in the thick-leaved species of 

 Euphorbia (e.g. E. biglandulosa and E. Myrsinites), where the protruding 

 tips of the tracheides often expand into more or less spherical vesicles. 

 Sometimes the terminal tracheides are altogether unusually wide, in 



which case they undoubtedly also act as 

 organs of water-storage (for details cf. Ch. 

 VIII.). 



As a rule the leptome does not extend 

 to the very tips of the bundle-ends, though 

 it may do so in exceptional cases. In the 

 bundle-ends of Angiosperms, according to 

 fig. 146. A. Fischer, 176 the sieve-tubes gradually 



Foliar bundie-end of Fieus eiastica, decrease in width and the sieve-plates 



consisting of two rows of spiral trach- . 



eides enveloped in a sheath of con- become more and more imperfect. The 



ducting parenchyma (parenchymatous , 



bundie-sheath) which is two ceiis thick companion-cells, on the other hand, retain 



on one side. . 



their ordinary dimensions, and may even 

 become somewhat enlarged (Fig. 147 b, c). The relation between 

 sieve-tubes and companion-cells, in point of size, which prevails in 

 the bundle-trunks or larger conducting strands of the leaf and stem, 

 is thus reversed in the bundle-ends. In the last resort, the common 

 mother-cells of sieve-tube segments and companion-cells cease to divide 

 altogether (Fig. 147 a). Both these terminal undivided elements, which 

 possess the abundant protoplasmic contents and large nuclei of genuine 

 companion-cells, and the above-mentioned unusually wide, but other- 

 wise normal companion-cells, are termed transitional cells by Fischer. 

 The physiological significance of these structures is obscure. Possibly 

 they transfer to the sieve-tubes protein-compounds manufactured in 

 the photosynthetic tissues. Fischer believes that they are themselves 

 the protein-manufacturing elements of the leaf. 



The photosynthetic organs of Conifers and Cycads are entirely 

 devoid of fine vascular ramifications. Here the uniform distribution of 

 water to the various parts of the transpiring mesophyll is ensured by the 

 development of a fringe of tracheal tissue along both sides of each 

 foliar hadrome-strand, the so-called transfusion tissue 177 (Von Mold) or 

 " border of tracheides " (De Bary). This tracheal fringe may take the 

 form of two lateral wings projecting into the adjacent photosynthetic 

 tissue ; or it may embrace the hadrome or leptome portion of the vein ; 



