29 



hrought into a niore conccntrrited solution tb.an was normal to them, they 

 showed an increase of intra-cellular pressure. If the experiment was re- 

 versed, the pressure decreased. These changes in osmotic pressure are 

 caused by the difference in concentration of the cell sap which may again be 

 considered as a result of chemical changes. If the cell comes in contact with 

 a solution too highly concentrated, it forms oxalic acid which acts strongly 

 Gsmotically. With Tradescantia, van Rysselberghe proved the presence of 

 malic acid in the normal sap and only in rare cases any traces of oxalic acid. 

 After the plant had been kept some days in strongly concentrated cane sugar 

 solution, oxalic acid was found in clearly appreciable amounts. The plant 

 gradually adjusts itself to the higher concentration of this medium, produc- 

 ing oxalic acid in order to increase the pressure of the cell sap. The acid is 

 supposed to be formed at the expense of grape sugar. The increased acid 

 content will act as a protective means against bacterial attacks. It is also 

 suggested by some investigators as a protective weapon against the attacks 

 of snails and leaf lice. 



Experiments with Tradescantia made in the opposite direction seem to 

 me to be very significant. If tissues from this plant were taken from the 

 highly concentrated solution and put into some strongly diluted solution, 

 precipitates of calcium oxid crystals were observed in the cell sap, thereby 

 initiating a decrease of osmotic pressure. When the plant was put back into 

 a stronger solution the oxalic crystals were seen to re-dissolve and result in 

 a new formation of acid. I found that part of the calcium oxalate crystals 

 disappeared during the sprouting of potato tul^ers which also may well be 

 ascribed to the increased formation of acid. 



Pfeffer^ also takes up this automatic regulation of the acid con- 

 tent since he calls attention to the frec[uent production of turgidity through 

 the organic acids combined with bases. Since this remains constant during 

 and after growth, the formation of acid must be hastened quantitatively in 

 correspondence with the volume increase of the cell and the dilution of the 

 cell sap thereby produced. Each unusual increase of turgor, as, for example 

 in the effort to overcome an opposing higher concentration, will be connected 

 with a corresponding increase in the acid production. Conversely, for exam- 

 ple in the Crassulaceae, the decrease of the acid content has been proved 

 with an increase in temperature and by illumination. In this sams sense the 

 experiments made by Charabot and Hebert- have succeeded. In the shade, 

 the quantity of combined organic acid increases very considerably. 

 The free volatile acids also increase. These are found in greater amounts in 

 etiolated plants than in others. The suppression of the inflorescences in- 

 creases in the leaves at the expense of the other organs. 



In considering predisposition and immunity, we have brought forward 

 the sugar content in addition to the examples of acid content. To what 



1 Pflanzenphysiologie, II Edition, Vol. I, p. 4S7. 



2 Charabot, Eug., et Hebert, Recherches sur 1' aeidite vegetale. Compt. rend, 

 hebd. 1904. CXXXVIII, 1714. 



