326 ASSIMILATION. 



been seen on page 197) there is always intermingled an incon- 

 stant aniouut of carbohydrates, or proper food-materials, etc. 

 At different stages in the life of a cell its protoplasmic matters 

 ma}' pass through considerable changes of form and structure, 

 as indicated in an examination of a i-ipening seed ; but under all 

 these varying conditions nitrogen in combination is never absent 

 from the living substance of the plant. 



858. For the formation of new protoplasmic matters in the 

 plant, supplies of nitrogen in an available form must be fur- 

 nished ; for healthful growth, these supplies must be adequate 

 in amount. 



859. Dissolved albuminous matters of various kinds are met 

 with in the sap of some cells. Tliis in manj- cases appears to 

 be, as will be shown later, a form in which their transport from 

 one part of the plant to another is secured. A small number of 

 these albuminous substances have been shown to be ferments, 

 which plaj' a verj- impoitant part in the nutrition of the plant. 



860. Although by far the greater part of the combined nitro- 

 gen of the plant exists in one or more of the combinations men- 

 tioned in Chapter XI., there is often to be detected a small and 

 variable amount as a nitrate ' (generally potassic) , and even as 

 a salt of ammonia. 



between certain groups of these bodies as tliey are represented in tlie animal 

 kingdom, dividing them into (1) albuminoiia matters and (2) their derivatives 

 or albuminoids (see Gorup-Besanez, Lehrbuch der Chemie, iii., 1874, p. 115). 

 Although the latter term, without the re.striction here noted, is in commou 

 use in vegetable physiology to designate these bodies, an objection can justly 

 be urged against its employment, on account of the more common use iu 

 botany of the word alhimen with an entirely different signification (see Volume 

 I. p. 14). 



In 1838 Mulder published the theory that all these bodies are practically 

 derivatives from one substance, termed by him proteine (from Trpwreiim, to be 

 first) ; but it was soon shown that this theory was erroneous, and it has been 

 generally abandoned. The term introduced by Mulder to designate the hypo- 

 thetical compound common to all these bodies has, however, been since em- 

 ployed to conveniently denote the whole class. In using the convenient term 

 protein bodies, or proteids, to designate the members of this group, it must not 

 be understood that the abandoned theory of Mulder is taken into account at all. 



' For the detection of nitrates the following test may be employed : To a 

 drop of tlie sap under examiniition add a drop of a solution of brucine, mix, 

 and tlien add a few drops of concentrated sulphuric acid, when, if a nitrate is 

 present, a red color will appear. Sprengel's reaction may also be used : One 

 part of phenol is dissolved in four parts of concentrated sulphuric acid, and two 

 parts of water are added. If a drop of this solution is added to a solid nitiate, 

 a reddish color is produced. On adding strong ammouia, the color turns green 

 and afterwards yellow. 



