326 ASSIMILATION. 



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

 stant amount 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 ripening 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. This in man}* 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 play a very important 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 1 (generally potassic) , and even as 

 a salt of ammonia. 



between certain groups of these bodies as they are represented in the animal 

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

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

 Although the latter term, without the restriction here noted, is in common 

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

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

 botany of the word albumen 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 TrpwTeuw, 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. 



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

 drop of the sap under examination add a drop of a solution of brucine, mix, 

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

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

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

 parts of water an- added. If a drop of this solution is added to a solid nitrate, 

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

 and afterwards yellow. 



