512 CONDITIONS AND MECHANICS OF GROWTH. 



in'i-ipheral layers. This pressure is obviously stronger the more elastic and the 

 tirnier are the peripheral layers; and the elastic outermost parietal layer of the 

 cell is of course adapted to exercise an especial reaction on the fluid in the interior 

 of the cell. But that a pressure exists both towards the interior and in the reverse 

 direction, in structui-es which have no cell-wall and consist only of protoplasm, 

 is shown by the fact that if rents are made in the outermost layer of myxomycetous 

 Plasmodia, fluid substances immediately pour out. 



It is indeed a matter of course that in a swollen, turgid cell the molecules of 

 the peripheral extended layers will be separated beyond the usual limit of cohesion, 

 and this is near to assuming that in the wddened interstices so formed fluid 



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materials are forced wdiich become firm the moment they are deposited, and then 

 resemble in every way the molecules they have driven apart. This intercalation 

 and hardening of constructive materials, which indicates an accompanying increase 

 in the bulk of the organized substances, is to be regarded as growth. We thus 

 obtain a conception of the mechanism of growth, which, though only hypothetical, 

 is in harmony with the external visible phenomena. We are led to it especially 

 by the fact that only cells which are turgid grow, whilst cells stop growing, even 

 although the necessary amount of fluid building material is present, as soon as 

 their turgidity diminishes. 



The turgidity of cells, that is, the presence in them of water necessary for their 

 swelling, is, however, only one condition of growth; a second condition, no less 

 important, is warmth. Without heat there is no growth. When in the temperate 

 zones, where the year is divided into summer and autumn, winter and spring, the 

 summer draws towards a close, and the days become shorter and shorter, when 

 during the long nights the soil loses more warmth by radiation than it gains during 

 the day, and when, too, plants become very much cooled, growth above-ground 

 entirely ceases, and the whole energy of the plants is concentrated, as we have 

 shown in previous sections, in changing itself into a chrysalis for the winter, in 

 withdrawing from the deciduous foliage the materials which can be employed 

 in the ensuing period of vegetation, and in lodging them in protected store-rooms. 

 During the winter, then, the cooled portions, unprotected against frost, rest, and 

 their growth is completely interrupted. At length winter is past, the last snow 

 has vanished under the breath of mild spring breezes; the hard frozen earth is 

 liberated from the bondage of the frost; everywhere new life stirs, buds swell, 

 trees adorn themselves with flowers and fresh foliage, the meadows become green, 

 seeds germinate, and the crops in the fields spring up vigorously, to the joy of the 

 farmer. On warm, sunny spring daj's everything grows with astounding rapidity: 

 on cool, dull days the increase is only small. If occasionally a relapse occurs, and 

 the temperature again sinks low, then the growth is wholly arrested. It has 

 been found that the increase of young herbaceous plants on two successive days had 

 sunk in consequence of a sudden storm and visitation of cold from 8 cm. to h cm. 

 There is no doubt that such a decrease of growth stands in causal connection with 

 the fall of temperature, and also that quick s^rowth is to be laid to the account of 



