38 MORPHOLOGY OF THE CELL. 



essentially on the quantity of water it has absorbed. But, however great may be 

 the quantity of water, and its consequent similarity to a fluid, the protoplasm is 

 nevertheless jiever a fluid ; even the ordinary dough-like, mucilaginous, or gelatinous 

 conditions of other bodies can only be very superficially compared with it. For 

 the living and life-giving protoplasm is endowed with internal forces, and, as the 

 result of this, with an internal and external variability which is wanting in every 

 other known structure ; its active molecular forces cannot, in short, be conipared 

 with those of any other substance \ The capacity which protoplasm has, in conse- 

 quence of the forces which become manifested in it, of assuming definite external 

 forms, and of varying these, as well as its capacity of secreting substances of 

 different chemical and physical properties according to definite laws, is the imme- 

 diate cause of cell-formation and of every process of organic life. 



The protoplasm of plants in a state of vital activity is generally very watery, and 

 shows on one side an internal diff"erentiation of its substance into layers and portions 

 diff'ering in their consistence and chemical nature ; on the other side it assumes definite 

 outlines, and becomes bounded by surfaces of determinate, and mostly very variable, 

 form. 



The internal differentiation of protoplasm is most commonly manifested by 

 an outer, hyaline, apparently firmer, but mostly very thin layer, enclosing the 

 inner mass, but in such a manner that the two remain in the most intimate con- 

 tact. Every portion of a protoplasmic body immediately surrounds itself, when it 

 becomes isolated, with such a skin (Hautschicht). Also in the interior a quantity 

 of fluid sap, which permeates its substance throughout, invariably becomes 

 separated in the form of drops (vacuoli) ; when the protoplasm is contained 

 in a growing cell, these vacuoli increase in proportion as the cell grows, and the 

 protoplasmic body becomes a sac filled with watery sap. One of the most common 

 internal differentiations of the young protoplasmic body, while constituting itself 

 into a separate individual, is observable in the formation of the nucleus. The 

 substance of the nucleus is at first indistinguishable from that of the rest of the 

 protoplasm, and its formation is essentially nothing but the collection of 

 certain particles of protoplasm round a centre, which is also usually the centre 

 of the whole protoplasmic body. Once formed, the nucleus (whose chemical 

 nature, as far as observation goes, is altogether very much like that of the pro- 

 toplasm) may become more sharply defined ; it may itself form a skin, and 

 vacuoli and granular formations (the nucleoli) may become separated in it. But 

 the nucleus always remains a part of the protoplasmic body; it is always imbedded 

 in it ; very commonly it becomes again dissolved, after a short existence, in the 

 protoplasm, i. e. its substance combines with it {e. g. in cells which frequently 

 divide, as on p. 14: in the sacs of the Characese the nucleus disappears altogether 

 when the streaming (Stromung) of the protoplasm begins). Another very common 

 differentiation of the substance of the protoplasm consists in single portions of it 

 becoming separated in a definite form and assuming a green colour, thus forming 

 chlorophyll structures, which, like the nucleus, not only arise out of the protoplasm, 

 but always remain as portions of the protoplasmic body. But since these require 



^ For further details on this point, &ee Book III ; also my Handbook of Experimental 

 Physiology, § 116. Leipzig 1865. 



