134 THE ROYAL SOCIETY OF CANADA 



having low emulsoid concentrations, suggesting the conclusion that the 

 size is attained by virtue of this condition. Borowikow, indeed, has 

 conceived the problem in this lights Working with Helianthns seed- 

 lings he has maintained that only under conditions conducive to the 

 hydratation processes of the colloids of the cell is the growth process 

 (stretching phase) possible. Conversely, he maintains that the con- 

 ditions which favor the increase of osmotic pressure within the cell 

 but which at the same time reduces the colloidal swelling, lower the 

 growth rate. 



The further questions, as to whether and how far the factor of 

 imbibition on the part of the protoplasm itself can play a rôle in 

 growth, remain quite unanswered, indeed have scarcely been ap- 

 proached. Among the conditions to be fulfilled by the material which 

 will permit such approach are the following. Sap-vacuoles must not 

 be present, thereby excluding an effective osmotic mechanism. The 

 absence of vacuoles does not of course result in the total absence of 

 osmotic pressure, but it does insure that the whole of the imbibition 

 pressure exerted by the protoplasm is directed outwardly against the 

 cell-wall. The manner of growth must also be such as to allow 

 accurate measurement. Of the plant materials which fulfill these 

 conditions, certain kinds of pollen have been found most useful. The 

 readiness with which pollen is frequently destroyed by wetting is due 

 to the great capacity of the protoplast for imbibition, e.g. that of the 

 cotton plant. For this reason principally it is necessary for the success- 

 ful initial growth of pollen tubes that the culture medium shall have 

 the capacity to withold water beyond that measure useful to the pollen, 

 the amount namely which will allow imbibition up to but not beyond a 

 certain quantum. Some pollen {Lupinus sp.) will begin growth in 

 distilled water, but will soon burst^ the pressure of the protoplast 

 becoming too great for the thickness of the cell-wall at the apex of the 

 pollen tube. The protoplast can, however, obtain sufficient water for 

 growth from a medium which at first withdraws water from it. Never- 

 theless, both the rate of subsequent growth and the total amount of 

 growth appear to be definitely related to the water-withholding 

 capacity of the medium. This capacity will be related to the imbibi- 

 tion capacity of the protoplast. If so, the introduction of substances 

 which, without measurably altering the osmotic pressure of the proto- 

 plast, do alter its imbibition pressure, should aflect its growth rate. 

 The following experiments were done to the purpose of determining 

 the facts in the case. 



iRorowlkow, G. A. Ueber die Ursachen des Wachstums der Pflanzen. 

 Bioch. Zeitschr. 48:230. 1913. 



»Lloyd, F. E. Ann. Rep. Dept. Bot. Res. Carn. Inst. Wash, for 1915. 



