58 MORPHOLOGY OF THE CELL. 



took this name, and this is indeed typical parenchyma ; but the 

 name rightly includes, as species or varieties, thicker-walled and 

 even solidified tissues composed of cells similar in other respects 

 to the type, as those in the hard endosperm of seeds. 



198. A counterpart name, Prosenchyma, was employed to 

 designate tissues formed of elongated cells, such especially as 

 wood-cells and bast-cells. These being usually thick-walled, 

 and those of tj'pical parenchyma thin-walled, this character was 

 brought into the definition ; that is, cells of prosencl^-ma were 

 said to be thick-walled as well as long and narrow, those of 

 parenchyma thin-walled as well as isodiametric. But this dis- 

 tinction does not hold out well. All fibro-vascular tissues are 

 thin-walled at first, and some remain so ; while portions of pure 

 parenclryma ma}- become thick-walled, firm and hard, or take on 

 every intermediate condition. So that prosenchyma may be best 

 held to denote tissue of the fibro-vascular system, and typically 

 that formed of wood-cells. 1 



199. An explanation of the mode of production, multiplica- 

 tion, and transformation of cells is deferred to a later stage. 

 Suffice it here to advert to the fact that even- phaenogamous 

 plant, originating in the seed, begins as an isolated cell, which 

 develops into a globular cluster of parenchyma cells, and grows 

 into the embryo or rudimentary plantlet, taking on the shape and 

 degree of development characteristic of its kind. In embryos 

 which are considerably developed in the seed, the axis and be- 

 ginnings of the leaves are already outlined or rudimentarily 

 indicated there ; in others the indication takes place in the early 

 stages of germination. 



200. From this if not from an earlier period development 

 is no longer homogeneous. A superficial la} T er of the common 

 parenchyma becomes distinguishable as the epidermis ; while in 

 an inner zone, or at special points, certain cells develop into ducts 

 and wood-cells (prosenclryma), and thus are initially delineated 

 the outlines of the systems or regions which are to characterize 

 the whole growth ; namely, taking a dicotyledonous embryo 

 for the type, an epidermal layer, a cortical layer, a fibro-vascu- 

 lar zone, and a medullary portion. As stem and root develop, 

 these primordial tissues complete themselves and have only to 

 go on growing, each after its kind ; but at the developing points 

 (apex of the stem and of the root), as also in special portions or 



1 " Zu dem Prosenchym im weitern Siune kbnnen wir auch die Gefasst 

 ziihlen " (Nageli : Beitrage, i. p. 2). 



