THE GROWTH OF THE CELL 



259 



under the microscope, doubling its amount in some circumstances in twenty 

 to thirty minutes, but we are quite ignorant how the protoplasm is formed 

 from the nutritive substances supplied to it ; we can only say that 

 we are dealing with a process of assimilation, or, more accurately, the 

 process of assimilation (compare Driesch, 1901). What we have hitherto 

 spoken of as ' assimilation ' has been the comparatively simple synthesis of 

 organic substances. The chloroplast synthesizes carbohydrates, but the carbo- 

 hydrates are no more like the chloroplast than the carbon-dioxide is ; they want 

 life, an essential characteristic of the chloroplast. We can only speak of assimila- 

 tion in the real sense of the term when the raw materials are transformed into a 

 living state, and this is what takes place when protoplasm grows, when, in other 

 words, new protoplasm is formed. The characteristic feature of the organism 

 is, more than any other, the way in which protoplasmic growth is accomplished. 

 When a crystal grows, it finds the same material already dissolved in the matrix; 

 the protoplasm, on the other hand, constructs itself out of substances unlike itself 

 but always as an addition to protoplasm there already. We are quite unable, at 

 present, to say how the process is carried out, since we are 

 quite ignorant what protoplasm itself really is. 



The newly-formed protoplasm must in some way or 

 other be incorporated in that already present ; in a word it 

 must headded to it or interpolated betweenits constituent parts. 

 To the question ' where does the protoplasm grow ? ' we 

 can give no definite answer, for it leads us at once to another 

 as yet unsolved problem, viz. the ultimate structure of the 

 protoplasm. The position we take up as to the various 

 theories of protoplasmic structure will determine what view 

 we take as to the mode of growth of protoplasm. It is 

 needless for us to follow out the various explanations which 

 have been given, since none of them have been generally _^ 

 accepted ; nor do we by accepting any of them gain any 'j t * 

 deeper insight into the nature of protoplasm. What applies ' -^ 



to protoplasm in general applies also to its members, the 

 nucleus and the chromatophores ; we see that they grow, 

 but we do not know where nor how. 



We may omit any discussion of the formation of proto- 

 plasm since, for the most part, it has no definite form. It 

 is a viscous fluid, whose form is determined frequently by 

 external forces. It is only when its external layer has attained a firm con- 

 sistence that we may speak of it as having actually morphological formation. 

 The cases in which that condition is reached, however, lead back to others 

 where the protoplasm has no such characteristic. 



We are much better, though still imperfectly, acquainted with the mode 

 of growth of the cell-wall. The chief difference between the protoplasm and the 

 cell-wall cannot be more clearly expressed than by saying that formation of 

 new protoplasm takes place only by addition to that already existing, whilst 

 a cell-wall may be formed where no cell-w^all existed previously ; the forma- 

 tion of a cell-wall necessitates the presence of protoplasm but not of another 

 cell-wall ; the protoplasm makes itself, the cell-wall is made by the protoplasm. 

 This dependence of the cell-wall on the protoplasm is the first thing that strikes 

 us in a study of its earliest beginnings, and we may start with a consideration 

 of the initiation of the cell-wall (Strasburger, 1898). Swarmspore formation 

 takes place in many Algae and Fungi. In the simplest case (Fig. 48) the contents 

 of the cell retreat from the wall, and finally escape through a crack in the wall into 

 the surrounding water in which the free mass moves about in the water as a 

 nake d 'swarmspore '. After a certain time the movement ceases, the swarmspore 



s 2 



Fig. 48. OedogoniiDn. 

 A^ two cells wIiosR 

 contents are in process 

 of transformation into 

 swarmspores ; B., free 

 swarmspore ( x 350). 

 After Pkingsheim, from 

 the Bonn Textbook. 



