286 ASSIMILATION. 



function. These cells are found chiefly in expansions upon or of 

 the axis ; of course, most commonly in ordinary leaves. But in 

 many cases the primary axis itself and the secondary and other 

 axes (branches) may have a considerable share of the proper 

 assimilating tissue of the plant. In some instances, for exam- 

 ple, in solid-stemmed and fleshy plants (as Cactacese), the whole 

 assimilative apparatus is to be found on the surface of axial 

 instead of foliar organs ; and the same is true of certain ligneous 

 plants specially adapted to desert conditions (e. g., Colletia). 



763. The development of the assimilative system in land 

 plants appears to have been controlled by two opposed factors ; 

 namely, (1) the advantage to be derived from exposure to air and 

 light, and (2) the disadvantage consequent upon too great loss 

 of moisture by evaporation. Even the most superficial examina- 

 tion of the tropical plants cultivated in our hot-houses reveals 

 the striking manner in which a balance has been struck between 

 these conflicting influences : the plants of warm jungles (e. g., 

 Scitamineae) having broad and long leaves suited to a humid 

 atmosphere, while the plants of parched sands (Cactaceae and 

 the like) are characterized by some protection against excessive 

 evaporation. In both these extreme cases the provision for a 

 certain amount of evaporation is, on the whole, seen to be 

 tributary to the essential work of all green tissue, namely, 

 assimilation. 



764. Proper exposure of the assimilating apparatus of a plant 

 to light is secured (1) by the shape and position of the assimilat- 

 ing organ, whether it be axis or leaf, and ('2) by the arrangement 

 in the organs of the cells themselves. Concerning the first, see 

 Volume I. ; in regard to the second, see this volume, page 159. 



765. Chlorophyll ] (^Xwpos, green, and <u'AAov, leaf). The term 

 chlorophyll, originally applied to the pigment rather than to the 

 substance which contains it, is now used indifferently to denote 

 the coloring-matter and the portions of protoplasmic mass which 

 are tinged by it. It is better, however, to designate the former 

 chlorophyll pigment, the latter, chlorophyll granules, or grains. 



766. In regard to the genesis of the chlorophyll granules 

 which are the essential constituent of the assimilative cells, the 



1 " Nous n'avons aucun droit pour nommer une substance connue depuis 

 longtemps, et a 1'histoire de laquelle nous n'avons ajoute que quelques faits ; 

 cependant nous proposerons, sans y mettre aucune importance, le nom de 

 chlorophyle, de chloros, couleur, et <j>6\\ov, feuille; ce nom indiquerait le r61e 

 qu'elle joue dans la nature " (Pelletier and Caventou : Journ. de Pharmacie, 

 Ui., 1817, 490), 



