130 BOTANY 
vapour is therefore carried away as soon as it passes 
out of the leaf, transpiration is again more vigorous. 
In a bright light, too, more water is given out from 
the leaf, because light promotes the general activities 
of the plant. On a warm day there is more transpira- 
tion, chiefly be cause the root is then more active. 
The potometer (Gk. potes a drink, and metron a 
measure) is an apparatus (Fig. 88) for measuring 
accurately the loss of water by transpiration. A leafy 
shoot is cut from a plant, and, to prevent air from 
getting into the vessels of the wood, its cut end is 
immediately placed under water. Working under 
water the operator now passes the cut end through a 
hole in a large rubber cork, through which also passes 
a glass tube bent at right angles. The cork is then 
quickly inserted in a suitable jar full of water. The 
pressing in of the cork drives some of the liquid of 
the jar into the bent tube, which in this way is filled 
with water. As the shoot transpires, the water is 
withdrawn from the horizontal arm of the tube, so 
that, by measuring the area of the leaves and the bore 
of the tube, and finding the length emptied in a certain 
time, the rate of transpiration per square centimetre 
may be found. Here again the effect of different 
atmospheric conditions and the rates of transpiration 
in different plants may be tested. The flasks are, of 
course, simpler to use, but do not permit of accurate 
measurement of small differences. 
The rate of transpiration may also be measured by 
actual weighing. Obtain a plant growing in a pot. 
Water it well, and, when it has stopped dripping cover 
the surface of the soil with a piece of cardboard suit- 
ably eut to encircle the stem. Weigh the whole 
apparatus, and, after the pot has stood for a few hours, 
weigh it again. The loss in weight will roughly 
indicate the amount of water transpired. There would, 
of course, be some addition to weight by photosynthesis 
