250 MICROSCOPIC FORMS OF VEGETABLE LIFE. 



pears to be formed by its solidification. In the interior of the 

 viscid mass, are commonly found vacuoles, which are distinguished 

 from the surrounding substance by their difference in refracting 

 power ; these, however, are not usually void spaces, but are cavi- 

 ties in the protoplasm occupied by fluid of a more watery con- 

 sistence ; and this " vacuolation" of the interior, which increases 

 until the cell-contents have almost entirely lost their original 

 viscidity and are of a more watery character, seems to take place 

 part passu with the consolidation of the exterior into distinct 

 membranous walls ; so that the development of a perfect cell out 

 of a rudimentary mass of endochrome, may be stated to consist 

 essentially in the gradual differentiation of its substance, which 

 was at first a nearly homogeneous viscid mass, into the solid 

 cell-wall and the liquid cell-contents. It is interesting to ob- 

 serve, at the very outset of our inquiry into the nature of Organ- 

 ization and Vital action, so characteristic an illustration of the 

 great law of Von Bar, already referred to (pp. 52, 55). 



149. Now among the Protophytes or simplest Plants, on the 

 examination of which we are about to enter, there are many, of 

 which every single cell is not only capable of living in a state of 

 isolation from the rest, but even normally does so ; and thus, in 

 the ordinary phraseology, every cell is to be accounted a ''dis- 

 tinct individual." There are others, again, of which shapeless 

 masses are made up by the aggregation of contiguous cells, which, 

 though quite capable of living independently, remain attached 

 to each other by the mutual fusion (so to speak) of their gela- 

 tinous investments. And there are others, moreover, in which 

 a definite adhesion exists between the cells, and in which regular 

 plant-like structures are thus formed, notwithstanding that every 

 cell is still but a repetition of every other, and is capable of living 

 independently if detached, so as to answer to the designation of 

 a " unicellular" or single-celled plant. These different conditions 

 we shall find to arise out of the mode in which each particular 

 species multiplies by binary subdivision ( 150) : for where the 

 pair of cells that is produced by the segmentation of the previous 

 cell, undergo a complete separation from one another, they will 

 henceforth live quite separately; but if, instead of undergoing 

 this complete fusion, they should be held together by the inter- 

 vening gelatinous envelope, a shapeless mass results from re- 

 peated subdivisions not taking place on any determinate plan ; 

 and if, moreover, the binary subdivision should always take 

 place in a determinate direction, a long narrow filament (Fig. 

 104, D), or a broad flat leaf-like expansion (Fig. 104, G), may be 

 generated. To such extended fabrics, the term " unicellular 

 plants" can scarcely be applied with propriety, since they may 

 be built up of many thousands or millions of distinct cells, which 

 have no disposition to separate from each other spontaneously. 

 Still they correspond with those which are strictly unicellular, 

 in the absence of any differentiation, either in structure or in 



