DEPARTMENT OF BOTANICAL RESEARCH. 61 



gelatine and agar, may swell in a manner not to be predicted, in the 

 present state of theories as to the structure of colloids. 



A varying amount of water is lost at all times by transpiration from 

 the growing parts of plants and may exceed the supply to such an 

 extent that a stoppage of growth or an actual shortening may ensue; 

 how far this is accompanied by a slackening or cessation of adsorption 

 of new material in protoplastic colloids can not be said at present. 

 Marked variations in volume of the plant, however, are caused by 

 such excessive water-loss. 



The expansion or enlargement feature of growth is therefore a com- 

 plex process, the principal energy for which comes from unbibition 

 and osmotic pressure of colloids in the earher stages of the cell. Osmo- 

 sis plays a larger part in the later stages of distension. The tissues 

 and embryonic regions of the plants tested in the experimental work 

 at the Desert Laboratory swell more in weak alkaline solutions than 

 in acid or neutral solutions. Studies of the daily course of growth 

 show that it is most rapid at the time when the aciditj'- of the sap is at 

 a minimum or when a neutral or alkaline condition has been reached. 

 High acidity with decrease of imbibition and excessive water-loss may 

 check or stop growth with consequent shortening of growing organs. 



The Effect of Age, Acids, and Alkalies upon Imbibition by Growing Regions and 

 Tissues of Plants, by D. T. MacDougal. 



The capacity of a developing organ for taking up water by osmosis 

 and imbibition is a feature which plays a very important part in 

 growth. Mature organs generally have a greater capacity than those 

 in an embryonic condition. The possibility of making measurements 

 of material at various ages and under different conditions was one of 

 the important advantages of using the cacti in studies on growth. 



Clean disks 12 mm. across were cut from the flattened joints of 

 Opuntia. Three of these were arranged in a triangle in the bottom 

 of a Stender dish and a triangle of thin sheet-glass arranged to rest 

 its apices on the three disks. The vertical swinging arm of an auxo- 

 graph was now adjusted to a shallow socket in the center of the glass 

 triangle, while the pen was set at zero on the recording sheet. Water 

 or a solution being poured into the dish, the course of the swelling 

 was traced on a sheet ruled to millimeters. 



That the amount of imbibition depended on the presence of certain 

 recognizable substances, not on the mechanism of the living cell, was 

 demonstrated by the fact that dried disks gave proportionate differ- 

 ences equivalent to those of living material. 



The average thickness of disks varied from 4 or 5 mm. in the case of 

 young joints to 18 or 20 mm. in mature ones. The apical parts of 

 joints showed greater capacity for absorption than the basal ones in 



