LECTURE XXXIII. 



MECHANICAL CAUSES AND EFFECTS OF THE GROWTH 

 OF CELLS AND ORGANS. 



When vegetable cells pass over from their young condition into that of more 

 rapid growth, or we may also say when organs pass out of the embryonic state into 

 that of elongation, the taking up of water into the interior of the cells plays a 

 particularly important part, A longitudinal section through a root or a shoot, which 

 takes in at the same time the growing-point and the older parts of the organ, shows 

 at once that with the increasing volume of the growing cells the water in their interior 

 also increases, and exactly the same is observed in isolated living cells of Algae and 

 Fungi ^ The increase in volume of the vegetable cells corresponds almost exactly 

 with the quantity of water passing into their interior. It has already been mentioned 

 that the very young cells of embryos and growing-points, as well as the youngest 

 organs, are entirely filled with protoplasm and cell-nucleus ; in proportion as they 

 increase in length, breadth, and volume generally, the water of the cell-sap increases, 

 and the mass of protoplasm becomes more and more insignificant : the at first 

 solid protoplasmic body becomes changed into a sac filled with water and which is 

 closely applied to the cell- wall, as shown in Fig. 352. The cell-wall at first, where the 

 increase in volume of the individual cell or of a multicellular organ is concerned, 

 continually increases in extent only, without obtaining essentially in thickness. Only 

 when the increase in volume of the cells has ceased does the subsequent growth in 

 thickness of the cell-wall begin : a fact of the highest significance for the theory 

 of growth generally, and of fundamental importance, since it implies essentially that 

 growth in surface of the cellulose walls takes place only so long as the latter are 

 still exceedingly thin and accordingly extensible : strongly thickened cell-walls are 

 no longer capable of increasing in extent. These facts may be directly established 

 by microscopic observation, and there can be no doubt as to their mechanical 

 significance; moreover Gregor Kraus has also demonstrated the fact mentioned 



' The contents of this lecture have in the main, but much more in detail than here, been already 

 published in the third and fourth Editions of my 'Text -book' under the title 'The Mechanics of 

 Growth,' and in cases where the special literature is not quoted here, I refer once for all to that 

 chapter. There, for the first time, the growth of the cells and tissues in combination was con- 

 sidered from the mechanical point of view. With respect to tissue-tension, the last part of my 

 ^Handbuch der Exp. Phys^ (1865) may be consulted. 



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