OUTLINES OF THE PHYSIOLOGY OF NUTRITION 217 



We know little or nothing at present about the 

 first formation of proteids in the plant. It is most 

 probable that, like the carbohydrates, they are first 

 formed in the leaves. In addition to carbon, hydrogen, 

 and oxygen, nitrogen and sulphur also enter into their 

 composition, while the nuclein or chromatine of 

 the nucleus further contains phosphorus. The three 

 elements last named are, as we have seen, taken up 

 in the form of various salts with the water absorbed 

 by the roots. It appears probable that the proteids 

 formed in the assimilating tissue are taken up by the 

 phloem-cells at the ends of the vascular bundles, and 

 it may be regarded as certain that they are conveyed 

 to other parts of the plant through the sieve-tubes. 

 We often find the proteids stored up in the form of 

 definite granules, such as the aleurone-grains, which 

 occur very generally in seeds (see p. 38). 



III. Absorption. We have now obtained some idea 

 of the chief nutritive processes of a chlorophyll-con- 

 taining plant, but we have still to inquire how the 

 various food-substances are absorbed. 



Eecent researches have proved that in the foliage - 

 leaves of land-plants the amount of carbon dioxide 

 absorbed in assimilation, or exhaled in respiration (see 

 below, p. 226), varies almost exactly according to the 

 number of stomata; where stomata are absent, the 

 amount is practically nil. 



Hence we see that the stomata are the organs by 

 which the gaseous interchange between the green parts 

 of plants and the atmosphere is maintained. In the 

 special case of assimilation, the carbonic acid enter? 



