THE PLANT 81 



The starch granules are attacked by diastase and converted 

 into sugar whenever the sugar in the sap falls below a certain 

 concentration. This is important, as only dissolved crystal- 

 loids can pass from one portion of the plant to another. 



In addition to the formation of carbohydrates, the leaves of 

 plants fulfil other important functions. 



Transpiration of water takes place chiefly through tho 

 stomata, but probably all exposed parts of a plant allow of 

 the escape of some aqueous vapour. The rapidity with which 

 water is evaporated from a plant depends upon several factors — 

 the temperature, the humidity of the air, the amount of light 

 received, and other circumstances. In consequence of the 

 evaporation of water from the leaves, a diminished pressure is 

 often set up in the upper parts of a plant, so that the rise of 

 water from below becomes easier. Thus a steady stream of 

 dissolved substances taken in by the roots rises into the 

 leaves, where, by transpiration, the water is largely evaporated, 

 and the dissolved substances are elaborated into nutritive 

 materials. If the soil water be very dilute, more water will be 

 evaporated than when the liquid is more concentrated. Thus 

 oats were found to evaporate 688 grammes of water for each 

 gramme of dry substance formed, when grown in a solution 

 containing 0'25 per cent, of nutritive substances, but only 

 515 grammes when in a 3 per cent, solution. 



It is not known how the formation of albuminoids takes 

 place, but it probably occurs in the leaves. The first step 

 appears to be the formation of amino-compounds from the carbo- 

 hydrates and nitrates, and the subsequent conversion of these 

 into albuminoids. It has been shown that with many plants, 

 leaves cut in the morning contain much less starch and nitro- 

 genous matter than similar leaves cut in the evening, indicating 

 that during the night a transference of the starch and albu- 

 minoids formed during the day takes place from the leaves to 

 other parts of the plant. The albuminoids probably have to be 

 converted into amides or amino-acids and the starch into sugar 

 before any movement from cell to cell can occur. 



B 



