120 WATER-ABSORPTION AND TRANSPIRATION. 



papers afford a delicate test for water-vapour. Place a thin leaf 

 between two cobalt-papers, and keep them flat by placing between two 

 dry pieces of glass. Notice which surface of the leaf gives off most 

 water- vapour, as shown by the change of colour. 



(o) The cobalt-paper method enables us to tell whether the stomates 

 are open or closed. Put two plants (e.g. Broad Bean seedlings) in 

 darkness and in the morning expose one (A) to light for about an hour, 

 keeping the other (B) in the dark. Remove and test simultaneously a 

 leaf or a few leaves from each plant, noting the time taken for the 

 paper in contact with their lower surfaces to turn pink ; there should 

 be a decided difference between A and B. 



(p) Cut off a leaf (e.g. Tropaeolum) and let it lie on the table until 

 it has become limp (not long enough to let it get dried up). Then cut 

 off a fresh leaf, and put the two leaves (each with lower side upwards) 

 below a cobalt-paper. Which leaf, the fresh or the wilted, reddens the 

 paper more rapidly ? The experiment shows that the stomates close 

 when the leaf withers. 



(q) Since evaporation causes cooling there must be a difference 

 in temperature between a fresh leaf (with stomates open) and 

 a wilted leaf (stomates closed). Wrap and tie round the bulb of a 

 thermometer (1) a leaf cut from a Tropaeolum, (2) a leaf still attached 

 to the plant (exposed to light), and note the readings of the two ther- 

 mometers, When the cut leaf has wilted, its temperature will have 

 risen (sometimes as much as 5 C. ). Repeat the experiment, keeping 

 the leaves in darkness ; the temperature of the two leaves remains prac- 

 tically the same (why ?). 



(r) That salt-solution causes the stomates to close may be shown, 

 without using the microscope, by means of cobalt-paper. Water a 

 Bean or other seedling in a pot with 0*5 per cent, solution of common 

 salt (1 gramme of salt in 200 c.cs. of water) for a few days. Test with 

 cobalt-paper leaves from this seedling (A) and leaves from a seedling 

 (B) watered in the ordinary way. The leaves of A redden the paper 

 much more slowly than those of B. Why is it that a iveak salt-solution 

 actually keeps cut or dug-up plants fresh ? Plants sent by post wil) 

 wilt much less if sprinkled with a weak solution of salt. 



154. Absorption and Excretion of Liquid Water by 

 Leaves. Unless there is a thick waxy unwettable cuticle on 

 their surface, leaves can absorb a certain amount of the rain 

 water falling on them. In Flowering Plants, however, this 

 capacity for absorbing water by the leaves is not nearly so 

 marked as in some of the lower plants (especially mosses and 

 lichens), and in many cases it is prevented by the very same 

 adaptations which the leaves possess for hindering excessive 

 transpiration (Art. 155). 



