OSMOTIC PRESSURE IN ANIMALS AND PLANTS 575 



the osmotic pressure in some way sets a limit to the amount of 

 hydrolysis of reserve colloids. 



Cavara and the Neapolitan physiologists examined the 

 changes in pressure taking place during the ripening of fruits. 

 In some cases, that of the grape for instance, there is a steady 

 rise towards maturation, so that the ripe sugar-laden fruit has 

 a pressure of about 30 atmospheres. In others a preliminary 

 rise is followed by a large fall in pressure. This corresponds to 

 a great increase in the size of the fruit. In such a case the 

 additional volume of the fruit appears to result from the 

 growth of each cell ; the vacuole solutes are therefore largely 

 diluted. 



An isolated instance of the secretion of practically pure 

 water by a leaf has been investigated by Dixon and Atkins. 

 The leaves of Colocasia antiquorum, when in a warm moisture- 

 laden atmosphere, give a stream of fine drops of water from the 

 tips . The drops may follow each other so fast that it is impossible 

 to count them . Twenty cubic centimetres or more of the liquid 

 have been collected from one leaf during the night. The ana- 

 tomy of the tip of the leaf has not been completely worked out 

 as yet, but the fact remains that water of immeasurably small 

 depression of freezing-point and of conductivity less than that of 

 Dublin tap water — a very soft upland water collected from 

 granite and similar rocks — is secreted from it . The solid residue 

 it leaves on evaporation is also extremely small. The interest 

 of the observation lies in accounting for the rapid passage of 

 water of such purity through the protoplasmic membrane of 

 the secreting cells, and more work remains to be done before a 

 satisfactory explanation can be given. 



Another problem studied by these methods was that of the 

 nature of the transpiration stream in woody stems. This had 

 generally been regarded as approximating to good drinking 

 water, though Fischer and others had shown that in the spring 

 reducing sugars were to be found in it. Dixon and Atkins at- 

 tempted to obtain the sap of the transpiration stream by 

 pressing the wood . It was, however, recognised that the sap was 

 loaded with the debris and consequently the solutes of the 

 living cells of the medullary rays and wood parenchyma. Short 

 lengths of the wood were then placed in a powerful centrifuge, 

 and a quantity of almost colourless clear sap was obtained. 

 On examination this was found to have a small depression of 



