464 MANUAL OF BOTANY 



discussed in detail in the present treatise. They include bodies 

 of varying degrees of complexity, some nitrogenous, others not. 

 Among the former rntuy be mentioned many compounds of the 

 amides with fatty acids, xanthin and bodies allied to it, and the 

 great group of the alkaloids. It is possible that many of these 

 may be of use to the plant in its processes of reconstruction, 

 but some are certainly of no value in this respect. The alka- 

 loids come under the latter category, for though they contain 

 combined nitrogen they cannot minister to the growth of the 

 plant. If a plant is supplied with them, but with no other form 

 of combined nitrogen, it is rapidly starved. The amidated fatty 

 acids, leucin, tyrosin, glycin, &c., on the other hand, can be 

 absorbed and utilised in the processes of nutrition. The latex 

 of plants frequently contains many of these bodies. Caoutchouc 

 is also present in some. 



Among the non-nitrogenous bye-products may be mentioned 

 the great variety of vegetable acids and many of the glucosides, 

 such as salicin, eoniferin, &c., which have a certain nutritive value 

 owing largely to the sugar they contain. Certain other products 

 derived from them may also be utiHsed in this way. Such bodies 

 may perhaps be best included among the reserve materials 

 already discussed. The vegetable acids, such as tartaric, malic, 

 citric, &c., are usually regarded as arising in the course of the 

 catabolic processes ; it is, however, possible that some of them 

 may be formed in the elaboration of food from the raw materials 

 absorbed, having thus their origin in anabolism. 



The bye-products include also a variety of aromatic substances, 

 such as tannin, phloroglucin, and aromatic acids, such as benzoic, 

 salicylic, &c., but the nature of the processes which give rise to 

 them is not well understood. 



Certain decomposition products of cellulose may also be 

 included here. The lignin and snberin, which are characteristi 

 of woody and corky cell-walls, arise in this waj'. During their 

 formation they can be removed by appropriate solvents, leaving 

 the cellulose skeleton which they are gradually replacing. These 

 differ from most of the bodies described in that they can be pro- 

 duced in the walls of cells that have lost their protoplasm, so 

 that their formation is independent of the latter. 



Finally, we may include here the odorous substances and 

 the colouring matters, except chlorophyll. Many colouring 

 matters are the products of the decomposition of the latter, 

 especially xanthophyll and erythrophyll, to which the autumn 



