280 GROWTH OF PLANTS 



proximity to and in precise alignment wdth the adjacent fibrils, thus exclud- 

 ing, in large measure, the colloidal, non-cellulosic materials from the cell 

 membrane. This procedure will account for the high degree of crystallinity 

 and the regular orientation of the mature Valonia membrane. ^^ Detailed 

 accounts of the visible aspects of cellulose and cell membrane formation in 

 both Halicystis and Valonia (Fig. 112) are being prepared for publication 

 in Contributions from Boyce Thompson Institute. 



These studies of bacteria, fungi, algae, and other types of cells from vari- 

 ous parts of the plant kingdom, represent the experimental procedures in 

 the attempt to understand the chemical and physical variations mvolved 

 in the formation and structure of plant cell membranes. 



When the cell membrane studies at the Institute terminated m 1940, 

 sufficient evidence had been accumulated to indicate the chemical hetero- 

 geneity and physical complexity of plant cell membranes in general; the 

 particulate state of the cellulose in many membranes from various parts 

 of the plant kingdom; the importance of non-cellulosic materials ui mem- 

 brane formation and function; the intimate colloidal associations of the 

 membrane components, which render their identification extremely diffi- 

 cult; the importance of using fresh (undried) material, whenever possible, 

 for experimental purposes; and the need to develop more precise methods 

 of fractionation and identification in order to determine the roles played 

 by the various membrane components in both native and processed mate- 

 rials. 



The physical aspects of cellulose synthesis in the living plastid, as well 

 as the behavior of membrane-forming materials during their period of 

 organization in the outer layers of the protoplasm, suggest a new field of 

 experimental approach to the study of the formation and structure of plant 

 cell membranes. The forces mvolved are frequently neither of a simple 

 molecular nor of a gross physical type. They fall rather into the colloidal 

 field of molecular aggregates of various dimensions and degrees of purity. 

 Surface coatings sometimes mask the chemical identity of such molecular 

 aggregates and determine their behavior in an electrical field. As data 

 accumulate, it becomes more evident that the properties of both living and 

 processed plant cell membranes must be interpreted in terms of the colloidal 

 systems which they represent, and not in terms of the molecular behavior 

 of any one component. 



The skill with which nature synthesizes and manipulates the plant cell 

 membrane materials will become more and more impressive as a clearer 

 understanding of the chemical and physical forces involved is obtained. 

 Until the accumulated information is more extensive than what we have 

 at present, we shall not be able to understand fully, e.g., the observed 

 similarities of properties of membranes of cells of very different types and 

 the differences in the make-up of membranes of closely related cells. To 

 future research is left the task of finding and establishing basic principles 

 of structure and composition common to all types of plant cell membranes, 



