Ways in Which Plants Increase in Size 



343 



a 



fully-adult cell, is sometimes surprisingly great, as the accom- 

 panying example well illustrates (figure 130). 



From these considerations it will be plain that the fully-adult 

 cell consists largely of water, with comparatively little solid 

 matter, in great contrast to the embryonic cell which is largely 

 solid. This is shown very clearly by the great 

 collapse of fresh plant-structures when dried 

 (for often they shrink away to a mere wisp of 

 their former selves), and also by weighings, 

 which prove that most fresh plant-structures 

 consist of more than 90 per cent water. A 

 plant as large as that shown in our figure, for 

 example, (figure 131), can be contained when 

 dried in the tiny vial beside it. The same thing 

 is true also of seedlings and the spring vegeta- 

 tion from buds; when the water is expelled, it 

 is found that the fully grown structure is not 

 only no heavier than the embryo or bud, but 

 even lighter in weight, — the loss of course be- 

 ing due to the removal of material by respira- 

 tion. Thus in general it is true that developing 

 structures gain weight, while growing struc- 

 tures lose it. 



That growth consists chiefly in swelling of 

 cells already laid down in development is 

 shown very beautifully by comparison of some 

 embryos with the seedlings that grow from them. If cross- 

 sections of embryos and seedlings be made in about the same 

 place, it is found on the average that although the cells differ very 

 greatly in size, their number is approximately the same, though 

 in one case they are tiny, squarish, densely packed and full of 

 substance, while in the other they are large, rounded, loosely- 

 arranged, and contain little but water. This separation of devel- 

 opment and growth is more common than one would suppose, for 



Fig. 130.— The com- 

 parative sizes of a 

 pith cell in the newly 

 developed and the 

 fully-adult condition, 

 as seen in optical 

 section. Traced from 

 accurate drawings on 

 a wall-chart by Frank 

 and Tschirch. 



