SIGNIFICANCE OF TRANSPIRATION 8i 



plastic, organ, the fundamental character of which, in its 

 normal form, is that it spreads a large area of chlorophyll to 

 the Hght. It is instructive to stand under a well-grown 

 beech and observe the architecture which brings its multitude 

 of leaves to the light, spread over the tips of branches and 

 twigs ; or under a maple with its tiered branches, between 

 which the light strikes to leaves borne far in towards the 

 trunk. There are scarcely any gaps in these leaf screens, 

 and yet little mutual interference. 



Of the materials of photosynthesis the water is of course 

 supplied from the soil, but the carbon dioxide comes from 

 the small quantity present in the atmosphere. This is 

 readily demonstrated by simple experiments. Sachs used 

 the formation of starch, which takes place in the leaves of 

 most dicotyledons during photosynthesis, and the presence of 

 which in small quantities is easily shown by iodine, to follow 

 the effect of various conditions. A plant kept in the dark 

 for a day or two loses the starch from its leaves, by conversion 

 into sugars and subsequent removal. Such a plant exposed 

 to light under normal conditions, forms, in the course of 

 a few minutes, sufficient starch to be demonstrated ; but if 

 it be exposed to light in an atmosphere deprived of carbon 

 dioxide, then no starch formation takes place. 



Transpiration. — It is essential, therefore, that the chloro- 

 phyll cells should be freely supplied with air containing 

 carbon dioxide ; but this, of course, means that gases in 

 general must be able to diffuse in and out of the leaf. 

 Thus oxygen diffuses into the leaf in the dark, supplying 

 the needs of respiration ; by day the excess produced by the 

 more active photosynthesis diffuses out. Much more 

 important consequences for the structure and behaviour 

 of the plant follow from the diffusion outwards of water 

 vapour. The leaf is a water-saturated organ ; it is freely 

 exposed to an atmosphere which has normally a marked 

 saturation deficit of water vapour ; air currents accentuate 

 the deficit by tending to prevent increase of moisture in the 

 neighbourhood of the leaf. Because the leaf is exposed 

 to light, the evaporation from it is increased with rise of 



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