222 LECTURE XII. 



amides are all richer in oxygen than proteid, and by the fact 

 (p. 210) though much stress cannot be laid upon it at pre- 

 sent that, in the absence of free oxygen, ammonia is evolved 

 by plants. We cannot say at present whether or not all the 

 amides are directly formed in this way. It is possible to 

 imagine that the more highly oxidised of them the xanthin- 

 bodies, for instance may be derived from others by further 

 oxidation. On the whole it appears probable that this is not 

 the case, but that the different amides are derived possibly 

 from different forms of proteid, or that the nature of the de- 

 composition may vary under the influence of different external 

 conditions, so as to give rise to amides sometimes of one kind 

 and sometimes of another. In connexion with this point we 

 must bear in mind Kossel's suggestion as to the origin of the 

 xanthin-bodies from nuclein. 



Turning now to the waste-products, we find that those 

 with which we have become acquainted in previous lectures 

 (oxygen, carbon dioxide, water) are such as are excreted by 

 the plant; but the majority of the waste-products are retained 

 in the tissues of the plant. Speaking generally we may say 

 that the excreta of plants are given off in the gaseous form, 

 though to this statement there are a few exceptions. It was 

 thought at one time that a considerable excretion of waste- 

 products in solution was effected by the roots, and de Can- 

 dolle went so far as to found upon this supposed excretion a 

 theory of the rotation of crops : but the researches of Bra- 

 connot and of Boussingault have conclusively proved that no 

 such excretion takes place. It is true that if the roots of a 

 land-plant be removed from the soil and be immersed in dis- 

 tilled water, small quantities of salts and even traces of organic 

 matter will be extracted from them (Knop), but no inference 

 can be drawn from experiments of this kind in which the con- 

 ditions are so abnormal. 



With regard to the nitrogenous waste-products we may 

 say that the more important of them are compound ammo- 

 nias. In some comparatively rare cases they are excreted 

 by the plant. Thus, the peculiar odour of certain flowers, 

 notably those of the May (Cratcegus oxyacantha], depends 



