THE SPECIFIC CONSTITUTION OF PROTOPLASM. 493 



dealing merely with a chemical compound. Nothing would be more natural 

 than a comparison with inanimate things whose outwardly perceptible features 

 are the expression of a definite chemical composition, i.e. of a certain grouping 

 of molecules and atoms which can be represented by a certain formula. But 

 although this comparison is allowable in general, yet there is an essential difference 

 between mineral and plant species. No formula can be given for the protoplasm 

 of a species of plant, and the structure of a protoplast cannot be compared with 

 that of a crystal. Each protoplast represents an organism which contains very 

 many chemical compounds. It is able to renew them when required, and to 

 modify their grouping as dictated by external stimuli. With these displacements 

 there must of course be a temporary alteration of structure, i.e. of the grouping 

 in the formed part of the protoplasm. But all these displacements and alter- 

 ations take place in each species according to the same plan. The same chemical 

 compounds, the same aromatic bodies, the same acids and the same alkaloids, &c., 

 can alone be demonstrated. The recently-formed parts agree with those already 

 present, and merely fill up the places previously assigned to them. This unalter- 

 able law of form which governs the working of the protoplasm in each species is, 

 therefore, dependent on some structure of the protoplasm which is beyond the 

 perception of our senses, and it is this which is termed the specific constitution of 

 the protoplasm. 



In the above-mentioned Desmids, which afford such especially instructive 

 examples, and in numerous other unicellular plants where all the formative 

 processes are carried on within a single protoplast, it is easy to see the connection 

 between the outward appearance and the specific constitution of the protoplasm. 

 It is more difficult in species where there is greater division of labour, a division 

 into manifold cell-forms and a gradual succession of diflPerent members. One 

 might compare the processes occurring in them with similar processes in mineral 

 species, which, with the same chemical composition present a great difference 

 in their outward form and appearance. Carbonate of lime, which forms the 

 mineral species calcite, appears in four kinds of crystal-forms, but they all belong 

 to the same system, and can be derived from one another. In the same way 

 the varied cell-forms and tissues, as well as the cotyledons, foliage, and floral- 

 leaves, which arise from the same plant in regular succession, are to be regarded 

 as members of the same system, succeeding one another in definite rule, although 

 the specific constitution of the protoplasm in the particular species undergoes 

 no change. 



At one time the idea was prevalent that there are two kinds of protoplasm, 

 idioplasm and cytoplasm. To the former was assigned the formative activity, 

 while the latter was regarded as merely a nutritive plasm or medium. Subse- 

 quently it was shown that in every protoplast there is a more definite part, termed 

 the nucleus, which dominates the whole, especially in the building and renewal 

 of the cell-wall, while it also takes a leading part in cell-division and multipUca- 

 tion. Thus the assumption that all formative processes are carried on through 



