92 



General Botany 



plants should be watered and allowed to become turgid ; water 

 should be poured into the holes in which they are placed before 



the soil is closed in around the 

 plants. It is customary also to 

 cover the plants for a day or two 

 with boards or paper covers so as to 

 reduce the transpiration. Main- 

 taining the water balance in trans- 

 planted plants may prevent the loss 

 of many of them and may save 

 weeks of delay in the maturing of 

 the crop. When herbaceous plants 

 are propagated by cuttings, pieces 

 of the stem a few inches in length, 

 with one or two of the uppermost 

 leaves, are taken and the lower 

 half of the cutting is put in wet 

 sand. In a few days, or weeks, de- 

 pending on the kind of plants, roots 

 are developed and a new plant is 

 estabhshed. The leaves are left 

 on the cutting so that some photo- 

 synthesis may go on. Most of the 

 leaves are removed, so that the 

 transpiration will not be sufficient 

 to dry out the stem. The cuttings 

 are kept moist, so that the absorp- 

 tion will be sufficient to keep all 

 the tissues turgid ; that is, to main- 

 tain the water balance. Cuttings of 

 woody plants that root with diffi- 

 culty are successfully started by painting the parts exposed to 

 the air with melted paraffine in order to keep them from 

 drying out. 



Fig. 49. Apparatus set up to per- 

 form an experiment to show the prin- 

 ciples involved in the maintenance 

 of the water balance in a plant. The 

 entire apparatus is filled with water, 

 and A and C are immersed in water. 

 The water is evaporated from the 

 porous cup B, and other water to 

 take its place is absorbed by osmosis 

 into the porous cup A. If the rate 

 of evaporation is faster than the rate 

 of absorption, the mercury falls in 

 the outer end of the tube C, as is 

 shown in the illustration. 



