404 Prof. H. Marshall Ward. The Relations let ween 



presence in the cell of water under pressure : that is to say, in a 

 turgid cell there is in the sap cavity sufficient water not only to 

 supply all the demands of the cell-walls and protoplasm, but to keep 

 them distended as well, and this to such a degree that the cellulose 

 walls, with their lining of protoplasm, are positively stretched in 

 opposition to the elastic resistance offered by the former. Recent re- 

 searches have proved that this excess of water is largely due to the 

 osmotic attraction exerted by the organic acids and their salts dis- 

 solved in the sap of the cell,**and since we have seen that the forma- 

 tion and destruction of these acids depend on the processes connected 

 with oxidation and respiration, we obtain a f uther glimpse into the 

 complicated correlations here concerned. For our purpose the im- 

 portant points are that, during active turgescence, the growing cells 

 tend to become very watery and their cell-walls to be thinned by 

 stretching, and this in spite of the activity of the protoplasm in 

 adding new materials ; while bodies such as soluble amides and 

 organic acids are being formed continuously and in relative and vary- 

 ing abundance, to undergo further changes in the never endiug 

 turmoil of metabolism, as already indicated. 



But it is evident that these processes of respiration, destructive 

 metabolism, and growth must sooner or later come to an end if the 

 stores of carbohydrates fail, since these are the substances which 

 ultimately supply the fuel for respiration, and which form the raw 

 materials by means of which new protoplasm may be constructed ; 

 and it is well known that the plant respires and grows to death if 

 placed in such circumstances that no new supplies of these substances 

 ;are possible. We must remember that we are concerned with normal 

 green cells, however, and we have now to consider the new set of 

 •events due to the assimilative action of the chlorophyll corpuscles to 

 which these cells owe their colour. It is not necessary to remind you 

 that this process of carbon assimilation^ consists in the coming 

 together of carbon dioxide and water in the green corpuscles, where, 

 by means of energy obtained in certain rays of sunlight, the molecules 

 of the carbon dioxide and water are torn asunder and eventually in 

 part rearranged; speaking generally, we may say that some of the 

 constituents (oxygen J) escape, while others (carbon, hydrogen, and 



* De Tries, " Ueber die Bedeutung der Pflanzensauren fur den Turgor der 

 Zellen" ('Bot. Zeitg.,' 1879) ; also Palladin, " Bildung der organischen Sauren in 

 den wachsenden Pflanzentheilen " (' Ber. d. Deutschen Bot. Gresellsch.,' 1887, p. 325). 

 Other literature will be noticed where necessary as we proceed. 



t For a general account of carbon assimilation, see Sachs, ' Lectures on the Phy- 

 siology of Plants,' especially pp. 296—323. 



X This oxygen is not active (see Pfeffer, ' Beitr. z. Kenntn. Oxydationsrorgange,' 

 p. 478). 



