Exercise II 



THE CELL WALL 



L Introduction.— Sperms, eggs and certain other cells of the 

 gametophyte of aiigiosperms are naked protoplasts and show no 

 evidence of a definite cell wall. Save for these particular ex- 

 ceptions, the zygote and all of the succeeding generations of 

 sporophytic cells which are derived from it are provided from 

 the beginning with a cell wall. Indeed the rigid and often mas- 

 sively-thickened cell Avail is frequently cited as a significant dis- 

 tinction between plants and animals; the cells of the latter are 

 either naked protoplasts or else possess thin and less clearly 

 demarcated walls, features which are obviously related to the 

 marked pliancy of many animal tissues. 



The form, relative thickness and chemical nature of the cell 

 wall, particularly the secondary wall, provide important and 

 definite characters for separating and classif.ying cell types in 

 higher plants. Consequently, a preliminary study of wall struc- 

 ture, together with a brief summary of the results of modern 

 research constitute a natural and necessary introduction to the 

 general problem of tissues and cell-types, which will be critically 

 discussed in the following exercise. 



IL Plasmodesmata.^ — The protoplasts of adjacent cells of a 

 wide variety of plant tissues have been shown to be intercon- 

 nected by delicate threads of cytoplasm, which are termed plas- 

 modcsmata (for a complete review, ef. Meeuse 1941). Protoplas- 

 mic connections seem to support the idea that the cell wall, at 

 least at certain stages in its growth, is an integral part of the 

 living system and not simply a "lifeless" excretion of the proto- 

 plast. Knowledge as to the origin and development of plasmo- 

 desmata is meagre but there is some evidence that additional or 

 ' ' secondarv ' ' connections may arise at diflPerent stages in ontogeny. 



1 For specijil technique to demonstrate plasmodosniata, of. Crafts (1931). 



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