ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 369 



1. Elongated cells of tubular and cylindrical, rarely prismatic 

 shape. Their position in relation to the surface of the assimilating^ 

 organ varies. Most commonly they are vertical, in which case they 

 are termed palisade-cells, less often parallel to the surface. "When 

 provided with arms or protuberances, they may be called branched 

 palisade-cells; funnel-cells, when the end nearest the surface is of 

 larger diameter than the other end. 



2. Tabular polyhedral oeZZs,^with or without infoldings of the wall. 



3. Isodiametric cells, with a tendency to rounding. 



4. Spongy parenchymatous-cells, of stellate form and much 

 branched. 



The cell- walls are sometimes furnished with simple pits, and are 

 usually thin and delicate. The chlorophyll-grains are as a rule 

 from two to six times more numerous in the paligade-tissue than in 

 the spongy parenchyma, from which the writer infers that the former 

 is in an especial manner the assimilating tissue of plants. The 

 assimilating cells frequently show infoldings of the cell-walls, as in 

 Pinus, the object being to increase the surface of cell-wall, and thus 

 provide room for a larger number of chlorophyll-grains. These 

 folds are so arranged as to facilitate to the greatest possible extent the 

 abduction of the products of assimilation. 



Dependent on the characters already mentioned, the author 

 classifies the various forms of assimilating tissue under ten types 

 arranged under the following heads: — (1) The assimilating tissue 

 serves also as an abducting tissue. (2) Both these kinds of tissue are 

 present, the products of assimilation passing out of the former into 

 the latter. (3) Besides these two kinds there is also a special con- 

 ducting tissue, through which the products pass in their way from 

 the assimilating to the abducting tissue. 



The spongy parenchyma subserves three distinct physiological 

 functions: — (1) It is peculiarly the transpiring tissue of the leaf, 

 (2) It is the conducting tissue. (3) In consequence of the larger or 

 smaller quantity of chlorophyll which it contains, it is an assimilating 

 tissue. 



Light, which is the most important external factor in assimilation, 

 while exercising a powerful influence on the arrangement of the 

 assimilating system, scarcely affects its anatomical structure. It 

 occasions the peripheral position of the special assimilating cells, 

 and, in dorsiventral organs, their production on the illuminated side. 

 The frequent occurrence of palisade-tissue is explained by the fact 

 that the position of elongated cells at right angles to the surface of 

 the organ favours the complete and intense illumination of the 

 organ. 



Every assimilating cell adjoins, at some part of its walls, the 

 aerating system or intercellular spaces, which also sei-ve to iireveut the 

 passing of the products of assimilation in uuadvantageous directions. 



The firmness of the assimilating tissue is secui-ed by a variety of 

 contrivances ; as the thickening of the walls of the palisade-tissue in 

 some species of Cycas, the columnar cells in Halcea, and the frequent 

 occurrence of branched sclerenchymatous cells among the green cells. 



