198 XV. THE LEAF VASCULAE BUNDLE ENDINGS. 



together internally into tufts. The products of assimilation, 

 therefore, are not passed laterally from cell to cell, but rather 

 make their way into the interior of the leaf. Here the tufts 

 of palisade cells join on to cells of the spongy parenchyma, which 

 often, at the point of junction, are broadened into funnel form 

 (sp\ Figs. 76 and 77), and their function may therefore be that of 

 receiving or collecting cells. The spongy parenchyma cells 

 which succeed these (sp", Figs. 76 and 77) may, from the same 

 point of view, be designated conducting cells. The spongy 

 parenchyma further contains air-cavities, which are in communi- 

 cation with the air-chambers of the stomata ; it is, therefore 

 also a ventilating or aerating tissue. It is also a transpira- 

 tion tissue, since from the surface of its cells specially copious 

 evaporation takes place into the intercellular spaces. Lastly, 

 the collecting and conducting tissue also is, by reason of its 

 chlorophyll contents, an assimilating tissue. The spongy paren- 

 chyma joins on to the parenchyma sheath of the vascular bundle, 

 to which they ultimately lead the assimilated materials, which are 

 partially conducted in the parenchyma sheath itself, partly in the 

 bast portion of the vascular bundle ; hence these last together 

 represent the conducting strings or conducting bundles. The 

 vascular bundles are, however, at the same time conducting 

 strings for water, which flows in the xylem, from which it is given 

 off to the surrounding tissue, and is also collected in the epider- 

 mis, which, in part, functions as a water reservoir. It is the 

 aggregate of this conducting tissue of the parenchyma sheath 

 around the vascular bundle in the form of nerve, or vein-paren- 

 chyma, 1 which, together with the strongly thickened " mechanical " 

 cells, promoting firmness, forms the tissue of the projecting ribs 

 of the leaf. The vein-parenchyma is continued into the ground- 

 tissue of the leaf-stalk, which, as we have seen in Ruta, is mainly 

 composed of conducting (to or fro) and mechanical elements. 

 In the petiole assimilating cells play only a subordinate part. 



1 It will be seen that three terms, viz., rib, vein and nerve, are used 

 almost indiscriminately by botanical terminologists to represent the same 

 thing. The term rib is correct so far as it refers to the mechanical nature of 

 the parts in question, acting just as do the ribs of an umbrella. The term 

 vein is correct so far as it refers to the conducting (water and food) function 

 of the vascular bundles contained in them. How far the term nerve may be 

 looked upon as correct in its implication must be left to the future to solve ; 

 but, under any circumstances, it must be considered far inferior in appro- 

 priateness to either of the other two. [ED.] 



