No. 454.] STUDIES ON THE PLANT CELL. 747 



method of growth a' process of eruption in contrast to Nageli's 

 conception of growth by intussusception. Increase in thickness, 

 comes with the laying down of successive lamellae inside the 

 older wall and is consequently growth by apposition. Caulerpa 

 is very favorable for such investigations and Noll's results greatly 

 strengthen the theory that a cellulose wall results from the 

 direct transformation of a plasma membrane in which carbo- 

 hydrate molecules gradually replace those of albuminous mate- 

 rial. Accordingly the cellulose wall is not strictly a secretion 

 and its growth is not by the intercalation of new molecules 

 among the old (intussusception) in a non-living membrane. 



The wide space in the interior of the filaments of Caulerpa 

 and some other members of the Siphonales is frequently crossed 

 by cellulose bars at various angles. These are at first strands 

 of protoplasm which become gradually filled with a carbohy- 

 drate material and finally solidified. Noll ('88a), while recog- 

 nizing that these structures may have "value in strengthening 

 the filament, believes that they are also the paths of metabolic 

 exchange; between the interior regions of the protoplasm and 

 the water outside the plant. They are surrounded by the 

 plasma membrane which in consequence presents a much greater 

 extent of surface to the water permeating the cell wall. 



It is plain that because of the constant movement of the 

 granular cytoplasm carrying with it the nuclei which change 

 their position in the cell, the outer plasma membrane is the only 

 portion of the protoplasm that can receive fixed stimuli for an 

 extended period. Consequently Noll regards this membrane as 

 the responsive or irritable region of the cell that reacts to the 

 stimuli which largely or wholly direct growth. Some of these 

 stimuli are well established. Thus it is light which directs the 

 formation of leaves and shoots. The behavior of Caulerpa in 

 relation to prominent stimuli (light, darkness, gravity, etc.) has 

 been studied by Noll (^%%b) and Klemm ('93). The latter author 

 believes that the response is due to the presence of foods or 

 other substances at certain points which make them especially 

 sensitive to the external stimuli. Injuries to a filarnent of the 

 Siphonales brings about an immediate flow of protoplasm to the 

 wounded part (Klemm,' 94), after which the plasma membrane- 

 is quickly repaired and new portions of the wall laid down. 



