282 GROWTH OF PLANTS 



and of explaining the procedures by means of which every given type of 

 plant cell produces the membrane suited to its vital needs. 



Summary 



(1) Problems relating to the formation and structure of plant cell mem- 

 branes have been approached, in the Boyce Thompson Institute for Plant 

 Research, through microscopic, chemical, and x-ray diffraction analyses of 

 cell membranes from various parts of the plant kingdom. 



(2) The first experiments dealt exclusively with the phenomenon of cell 

 enlargement in root hairs, in the membranes of which cellulose is either 

 absent or only sparingly present. Their plastic properties are determined 

 by the colloidal mixtures of non-cellulosic materials. 



(3) Cotton fiber studies began with the more general aspects of their 

 development from epidermal cells of the seed coat and resulted in additional 

 information concerning their origin from cells which had divided subse- 

 quent to fertilization; their nutrition through the basal connection with 

 the seed and not from "boll sap"; and the spiral fibrillar structure of their 

 secondary lamellae, as shown in polarized light and by means of x-ray 

 diffraction. The latter studies were made possible by the presence of crys- 

 talline cellulose in the fiber membranes in addition to colloidal non-cellulosic 

 materials. 



(4) Failure to obtain information concerning the fine structure of the 

 fibrils of the secondary lamellae led to studies of less highly differentiated 

 cellulose-forming cells including bacteria, fungi, and algae. 



(5) As a result of improved microscopic techniques, designed to observe 

 the cellulosic and non-cellulosic portions of Acetohacter xylinus, crystalline 

 cellulose was identified for the first time in the protoplasm of young cotton 

 fibers. It was in the form of ellipsoids, approximately 1.0 X 1.5 ^i in size, 

 which were named "cellulose particles" and were identified with Stras- 

 burger's "microsomes" and Wiesner's "dermatosomes." 



(6) Single rows of these cellulose particles are arranged end to end to 

 form the fibrils of the cotton fiber. The process takes place in the outer 

 regions of the living protoplasm and is followed by the deposition of spirally 

 arranged fibrils in a matrix of non-cellulosic colloidal material, to form the 

 secondary lamellae. Both pectic material and protein are present in this 

 matrix. Although the lamellate structure of the secondary wall indicates 

 the periodic deposition of wall materials, evidence was obtained to indicate 

 that this is not a daily periodicity; hence the lamellae do not represent 

 "daily growth rings." 



(7) In cells such as the green alga (Halicystis) fibrils are not formed 

 and cellulose particles in the successive lamellae are in more or less random 

 arrangement. In the Avena coleoptile, end-to-end bondage of cellulose 

 particles is not strong enough to prevent separation during cell elongation. 

 The stretching of the non-cellulosic membrane materials leaves the cellu- 

 lose in ring-like bands along the extended membrane. 



