STRUCTURE OF PLANT CELL lOJ 



and called it protoplasm ' This term was then extended 

 to the essentially similar substance ' found in animals. 

 The parallelism between the units of, protoplasm, each 

 with its nucleus, in animals and plants was soon recog- 

 nised, and the term cell gradually came to be used in 

 its modem sense as applying primarily to these living 

 units, which may or may not have definite cell walls. 

 In plants, however, as we have seen, they have walls 

 in the vast majority of cases, and the term cell is still 

 applied not only to the living cytoplast-nucleus unit, 

 but also to the wall and cavity after the protoplasm 

 has disappeared. 



The adult living plant cell is generally characterised 

 not only by the possession of a cell wall, but also by 

 a large central vacuole or space filled with a watery 

 liquid (cell sap), enclosed within the cytoplasm (Fig. 6, C). 

 So large is this in proportion to the whole space within 

 the cell that the cytoplasm is often a mere thin layer 

 lining the wall. The nucleus, which always remains 

 surrounded by cytoplasm, is embedded in this film 

 and bulges it out into the vacuole (Fig. 6, C). Vacuoles 

 are by no means the monopoly of plant cells — they 

 frequently exist in those of animals — but animal cells 

 as a class are not characterised by the possession of 

 a large central vacuole, the formation of which depends 

 on the mode of nutrition and growth of the typical 

 plant cell. The layer of cytoplasm next the vacuole 

 " vacuole wall "), like the external layer [ectoplasm ^) 

 next the cell wall, is an exceedingly thin film — a gel 

 membrane, such as we saw is usually formed at the 

 boundary of a colloid sol (Fig. 7). 



Embryonic (Meristematic) Cells.— In the higher plants 



■ Greek irptorog, first, and rrkdana, thing formed or moulded, as 

 from clay or wax. 

 ' iicrds, outside. 



