101 



way of a reduced leaf area, protective coverings to the leaf, 

 and so on. That such arrangements may be very effective is 

 indicated by experiments carried out in Europe which have 

 shown that the amount of water transpired by evergreen 

 conifers is frequently only iVth of the amount transpired by 

 deciduous dicotyledons, a result which is not surprising 

 when we consider the small area of leaf-surface exposed to the 

 air in the narrow coniferous needles, the strongly thickened 

 cuticle, the stomata sunk below the surface of the needles and 

 thus protected from air-currents, and the presence of scler- 

 enchymatous tissue beneath the epidermis which, as a rule, 

 surrounds and protects the thin- walled chlorophyll- contain- 

 ing parenchyma of such needles. Again, owing to their deep- 

 going root-systems some trees are able to tap a perennial 

 water supply and can thus produce fresh foliage and transpire 

 actively in the hottest, dryest season of the year. 



92. That a carbohydrate, which Assimilation. 

 is usually starch, is formed in the green leaves of a healthy 

 plant exposed to sunlight, from water and the carbon dioxide 

 of the air has already been pointed out. During this process 

 oxygen is evolved, the volume of oxygen given- off being 

 approximately equal to that of the carbon dioxide absorbed. 

 If a green plant is placed in an air-tight jar in an atmos- 

 phere from which the carbon dioxide has been removed, 

 no starch is formed in the leaves, thus indicating that 

 the carbon dioxide of the atmosphere is essential for the 

 process. If the cut stem of a green aquatic plant is placed 

 in a cylinder of water in the sunlight, bubbles of gas will be 

 found to escape from the intercellular spaces at the cut surface 

 which can be easily collected and proved to be nearly pure 

 oxygen. By counting the number of bubbles given off per 

 minute, an idea can be formed of the rate at which carbon 

 dioxide is being absorbed and starch manufactured. That 

 light is essential for this assimilation is shown by the fact 

 that no starch is formed in that part of a green leaf which is 

 covered with tinfoil, or otherwise protected from the light. 

 On the other hand it can be shown by experiment that all the Rays of Light 

 rays of light are not of the same value for this process. If, for re ^ uired 

 instance, a bright solar spectrum is projected on a green leaf 

 for several hours, the leaf then being decolourised with alcohol 

 and treated with a solution of iodine as before described, the 

 blue-black colour, indicating the presence of starch, will be 

 found to be most intense in that part of the leaf which was ex- 

 posed to the red and orange rays. The same thing can also 



