ASSIMILATION. 197 



and transformation, the protoplasm abandons them, the portion which 

 is not decomposed being constantly attracted onwards into forming 

 and growing parts, where it incites new development. For this 

 azotized matter has the remai-kable peculiarity of inducing chemical 

 changes in other organic products, especially the neutral ternary 

 bodies, causing one kind to be transformed into another, or even the 

 decomposition of a part into alcohol, acetic acid, and finally into 

 carbonic acid and water (as in germination, &c.), — itself remaining 

 the while essentially unaltered. 



356. The constant attraction of the protoplasm from the com- 

 pleted into the forming parts of the plants explains how it is, that 

 so small a percentage of azotized matter should be capable of 

 playing such an all-important part in the vegetable economy. It 

 does its work with little loss of material, and no portion of it is fixed 

 in the tissues. At least, the little that remains in old parts is capa- 

 ble of being washed out, showing that it forms no integral part of 

 the fabric. This explains why the heartwood of trees yields barely 

 a trace of nitrogen, while the sap-wood yields an appreciable amount, 

 and the cambium-layer and all parts of recent formation, such as the 

 buds, young shoots, and rootlets, always contain a notable proportion 

 of it. This gives the reason, also, why sap-wood is so liable to decay 

 (induced by the proteine), the more so in proportion to its newness 

 and the quantity of sap it contains, while the completed heart-wood 

 is so durable. The azotized matter rapidly diminishes in the stem 

 and herbage during flowering, while it accumulates in the forming 

 fruit, and is finally condensed in the seeds (which have a larger per- 

 centage than any other organ), ready to subserve the same office in 

 the development of the embryo plant it contains.* 



357. When wheat-flour, kneaded into dough, is subjected to the 

 prolonged action of water, the starch is washed away, and a tena- 

 cious, elastic residue, the Gluten of the flour, which gives it the 

 capability of being raised, remains. This contains nearly all the 

 proteine compounds of the seed, mixed with some fatty matters 

 (which may be removed by alcohol and ether) and with a little 

 cellulose. The azotized products constitute from eight to thirty 

 per cent of the weight of wheat-flour : the proportion varies greatly 



* The cotyledons of peas and beans, according to Mr. Rigg, contain from 

 100 to 140 parts, and the plumule about 200 parts, of nitrogen, to 1,000 parts of 

 carbon. 



17* 



