276 GROWTH OF PLANTS 



cellular organs or regions in the protoplasm which elaborate cellulose to be 

 used in the construction of the cell membrane. These proved to be cellu- 

 lose-forming plastids. The mechanism of formation of cellulose in the 

 chloroplasts of Halicystis has no apparent points in common \vith the 

 mechanisms of starch formation as it has been observed in many types 

 of plant cells. It operates by the successive formation of cellulose rings 

 of uniform thickness and increasing diameter, as the plastid enlarges. 

 These rings, at first in a liquid state, then gel-like, and then solid, finally 

 fragment to form mercerized cellulose particles. When the period of 

 cellulose synthesis comes to a close, the plastid membrane disintegrates and 

 the cellulose particles, with their coating of colloidal plastid plasma, are 

 set free in the outer regions of the cytoplasm of the cell. They are later 

 deposited directly, with their associated materials of plastid and cyto- 

 plasmic origin, to form a new lamella of the cell membrane. The newly 

 deposited lamellae are green due to the presence of the chlorophyll of the 

 plastid plasma (see Fig. 109). 



After observing this phenomenon of cellulose formation in the chloroplasts 

 of Halicystis, a careful study of the protoplasm of the cotton fiber was 

 resumed, with the result that colorless plastids, which were performing a 

 similar function of cellulose formation by a similar process of ring forma- 

 tion and fragmentation, were found. These plastids had been seen and 

 photographed previously in many living cotton fibers. Their function in 

 the cell had been obscured by the slight differences in refractive indices of 

 the plastid plasma and the cell plasma which surrounds them (Fig. 110). 

 They had been observed and considered to be vacuoles in the young fibers. 

 Once removed from the fiber, however, the stages of cellulose ring formation 

 and fragmentation (Fig. Ill) were clearly visible.^'' 



The chloroplasts of Valonia were found to be engaged in the elaboration 

 of cellulose by a different process from that observed in the chloroplasts of 

 Halicystis and in the colorless plastids of the cotton fiber. Following the 

 formation of a single cellulose ring in the very young plastid, a cellulose 

 fibril begins to form in the plastid plasma. These fibrils attain great length 

 and, in the mature plastid, are coiled tightly within the plastid membrane. 

 When the membrane of the mature plastid disintegrates, the coiled cellulose 

 fibril is freed and straightens. At this stage it can be disintegrated into 

 cellulose particles with slight pressure. Continued study of the early stages 

 of cell membrane formation has indicated, however, that in the living cell 

 the fibril is deposited directly in the lamella of the membrane after it has 

 uncoiled from the plastid. Observations show that it is deposited in close 



Figure 109. Column A, Stages in development of the chloroplast of Halicystis showing 

 cellulose ring and cellulose particle formation (1540 X). Column B, Stages in starch 

 formation in the chloroplasts of the cotton plant (1540 X). Column C, Cellulose particle 

 formation takes place in the colorless plastids of the cotton fiber by a process of successive 

 ring formation and fragmentation essentially similar to the mechanism of cellulose 

 formation in Halicystis (1540 X). 



