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LECTURE XXI 



THE GROWTH OF THE CELL 



The most simply organized plants are unicellular, and the microscope 

 shows us also that in the most complicated forms cells and their derivatives 

 constitute the units out of which these plants are entirely built. The cell is 

 thus to be considered as the fundamental constructive unit of the plant (and 

 incidentally it may be mentioned of the animal also). Any attempt to in- 

 vestigate the growth and formation of the plant will thus naturally commence 

 with a study of cell-structure. We will assume that all the conditions necessary 

 for the development of the plant have been fulfilled and that the determining 

 external factors remain constant. 



What do we understand by the terms ' growth ' and ' formation ' ? It is 

 obvious in the first place that the growing cell increases in size, but not every 

 increase in size is a case of growth. If we place some seeds in water we at once 

 observe a rapid increase in size owing to the enlargement of the individual cells ; 

 but it is due merely to the introduction of water into the organic substance, 

 to a process which we have termed imbibition. If we place the swollen seeds 

 in the air, the water evaporates and the seeds regain their original dimensions. 

 If we place the cell of an alga which has been plasmolysed by cane sugar (Lecture 

 II) in water, the cell increases in size owing to the absorption of water, but the 

 mode of absorption in this case differs from imbibition. Excess of water is taken 

 into the vacuole, but it induces no separation of the individual particles of the 

 wall and of the protoplasm. Generally speaking, this increase in size, owing 

 to turgor or imbibition, produces changes which are temporary, while in true 

 growth the changes are permanent. ' Growth ' is for the most part accompanied 

 by an increase in volume, but there are cases in which increase in one direction 

 is accompanied by a decrease in another. In the latter case, elongation can take 

 place without any change in volume ; still we may also speak of it as growth if 

 the change be permanent. 



We might speak of ' formation ' equally well when the organism forms 

 cells and when the cell has a specific shape in relation to the life-conditions. 

 But so interpreted ' formation ' would be no subject for science. The 

 causes of cell-formation we cannot indeed ascertain because we have never 

 seen non-cellular organisms (Sachs uses the term ' non-cellular ' in quite 

 another sense from this. * Lectures on Plant Physiology ') — only on theoretical 

 grounds we suppose their pre-existence (or also present existence) and regard 

 them as the simpler precursors of the cell. But if in speaking of * formation ' 

 we think of changing of shape then we acquire a notion of some fertility. 

 A changing of shape may happen alike in swelling and in osmotic enlargement 

 as well as in growth. But all these processes might go on also without 

 a changing of shape, if, for instance, the body simply became larger without 

 any alteration in its proportions ; in this case we would not speak of a change 

 of shape. 



We have already discussed the most important constituents of the cell 

 and their interrelationships elsewhere (Lecture I). Two of these only are of 

 interest to us at present, viz. the protoplasm and the cell-wall, and we will 

 attempt now to deal with their growth and form. We will begin with 

 the protoplasm, the essential living substance, and by far the most important so 

 far as our problem is concerned. Unfortunately, our knowledge of the growth of 

 protoplasm is very limited; we know little more than the mere fact that protoplasm 

 grows. It is possible to observe in many cells the actual increase of protoplasm 



