SECKETION". 579 



This seems borne out by the universal presence of these nitrogenous 

 compounds in actively vegetating cells, in roots, parts of the flower, in 

 cambium, &c., as well as in green organs. That the crude sap is found 

 to contain uncombined ammoniacal salts high up in the stem in spring 

 may result from the activity of the currents of fluid allowing part of them 

 to flow on undecomposed, while a part only is assimilated in the roots. 



Proteinaceous matters, it may now be stated with some confidence, 

 when not directly absorbed, originate in the colourless protoplasm, from 

 the decomposition of sugar and ammonia salts, in the same manner aa 

 starch is formed in the chlorophyll under the influence of light. Pasteur, 

 as we have seen, induced the formation of protoplasm in yeast-cells by 

 supplying them with a saccharine solution and a nitrate or ammonia salt. 



An animal of the simplest organization not only produces heat through 

 respiration, and exhales carbonic dioxide, but a certain portion of the 

 albumen it contains is modified by the respiratory combustion into a crys- 

 talline nitrogenous compound (urea}. In the case of plants, asparayin, 

 an amide, is formed in the darkness as a result of the general respiration, 

 and this is as easily transformed into aspartate of ammonia as urea into 

 its carbonate (Boussingault). This asparagin is a modification of the 

 albuminoid matters stored up in the cotyledons, formed when a transfer 

 of these matters is required, as in germination. 



Our space compels us to restrict this Section within narrow limits, and 

 we are obliged to omit any special reference to the application of these 

 generalizations to the explanation of the facts of Agriculture*. The 

 student is recommended to study works on physics and organic chemistry, 

 as vegetable physiology is daily becoming more and more a subject for the 

 physicist and chemist ; and without a knowledge of the subjects treated 

 of by students of these sciences, progress in vegetable physiology is im- 

 possible. What is specially wanted are experiments and analyses showing 

 precisely what physical and chemical changes go on in the several parts 

 of the plant at various stages of growth and their rationale. 



Sect. 7. SECRETION. 



At the commencement of the periods of activity of plants, as 

 when they shoot up from seeds, or when the new shoots are pushed 

 out in spring, the whole product of the elaborating processes is 

 devoted to the formation of new structure, to growth. As the 

 season advances, the cell-forming activity slackens, the permanent 

 tissues become consolidated by the formation of secondary deposits, 

 and the parenchymatous tissues appear loaded with accumulated 

 products of assimilation, such as chlorophyll, starch-granules, &c., 

 which in annual plants are subsequently consumed in the matura- 

 tion of the seeds, and in perennials are gathered together in autumn 

 and stored up in those tissues which are to carry on the develop- 

 ment in the succeeding season. 



* The student will find a useful summary of chemical science applied to 

 agriculture in a little work called ' How Crops Grow ' (MacMillan). 



