24 I The Process of Evolution 



tails together and their hydrophilic heads pointed toward the 

 hydrated protein strands, which have their long axis at right angles 

 to that of the lipid molecule?. Particularly in free-living cells and 

 cells in culture, the property of pinocytosis may be seen: By the 

 rapid and constant extrusion and withdrawal of minute pseudopod- 

 like extensions of their surface, the cells may ingest water and other 

 molecules. 



Free cells and cells in tissues seem always to have additional 

 layers outside the plasma membrane. In the animal cell, these 

 layers are composed largely of molecules of proteins and sugars, and 

 their integrity depends upon the calcium balance of the cell en- 

 vironment. Plant cells are rather different in two respects. First of 

 all, the outer layers (or cell wall) of plants are composed mainly 

 of carbohydrate molecules. Glucose residues arranged in long chains 

 form the most important constituent, cellulose. Other carbohydrates, 

 as well as fatty acid substances ( suberin, cutin, etc. ) , also are asso- 

 ciated with this wall. Contiguous cells in tissues are cemented to- 

 gether by a middle lamella, which is pectate in nature and also 

 dependent upon calcium for rigidity. The second important diflfer- 

 ence between plant and animal cells is that cells in plant tissues are 

 in organic connection through strands of cytoplasm called plasmo- 

 desmata. By means of electron micrographs, most instances of so- 

 called plasmodesmata in animals have been shown not to involve 

 continuity of protoplasm. That is, across the strands that look like 



Fig. 2.1 I Diagram of a general- 

 ized cell with parts as seen under 

 the electron microscope ( some 

 components enlarged or simplified). 



