218 HOW PLANTS GROW. 



Construction and Contents. — The typical cell consists, then, of 

 the following parts : — 



I. — A membranous boundary, the Cell-wail, consisting of 

 Cellulose (CtjHioOs), coloured blue by iodine and sulphuric acid. 



2. — The Protoplasfn, a semi-fluid, hyaline substance, consisting 

 of various organic materials, among which the albuminous (nitro- 

 genous) are never absent. The chief constituent is Froteifi, made 

 up of carbon, hydrogen, oxygen, and nitrogen, with a little phos- 

 phorus and sulphur, and a varying proportion of water. Proto- 

 plasm easily coagulates, has a very high absorbing power for 

 water, resists the passage of colouring matters, or salts, dissolved 

 in water, is coloured yellow-brown by iodine, and pink by sugar 

 and sulphuric acid, possesses powder of contractility, and is the 

 origin of all new development. A number of very small "gra- 

 nules " are imbedded in the protoplasm. 



3. — The Primordial Utricle. This is a definite boundary of 

 the protoplasm, similar in composition, but clearer, owing to the 

 absence of " granules." It lines the cell-wall where there is one, 

 and more or less bounds the granular protoplasm when there is no 

 wall. It is not sharply defined on the inside, but passes insensibly 

 into the protoplasm. 



4. — The Nucleus. A darker, more or less rounded part of the 

 protoplasm, in the centre at first, but afterwards often seen nearer 

 the wall. This is because in the young state the protoplasm 

 usually fills the cavity, but later on becomes merely a delicate 

 parietal layer, carrying the nucleus with it. The nucleus will 

 often creep about in a long cell like an Amoeba, and in so doing 

 causes the shifting bands, or " threads," of protoplasm, which it 

 drags after it. This may be seen very well in the cells of the 

 hairs of Tradescafitia (spider- wort). One or more " nucleoli," or 

 smaller nuclei, may often be seen within the nucleus, and the 

 latter may be usually detected, with careful focussing, by its being 

 clearer than the surrounding fluid. 



To detect the utricle as apart from the cell-wall, add a little 

 alcohol. This coagulates the protoplasm, and separates it, with 

 its utricle, from the wall. This can be made still more visible by 

 adding carmine, which colours the separated mass, leaving the 

 wall uncoloured. 



