PEDIA8TEEJE ; CONFEEVACE.E 



569 



curved outwards ; in a neighbouring pool every specimen may have 

 it curved inwards ; and in another it may be straight. The cause 

 of the similarity in each pool no doubt is that all its plants are off- 

 sets from a few primary fronds.' Hence the universality of a in- 

 particular character in all the specimens of one gathering is by no 

 means sufficient to entitle these to take rank as a distinct species ; 

 since they are, properly speaking, but repetitions of the same variety 

 by a process of simple multiplication, really representing in their 

 entire aggregate the one plant or tree that grows from a single seed. 

 Almost every pond and ditch contains some members of the 

 family Confervaceae : but they are especially abundant in moving- 

 water, and they constitute the greater A 

 part of those green threads which 

 are to be seen attached to stones, 

 with their free ends floating in the 

 direction of the current, in every 

 running stream, and upon almost 

 every part of the sea-shore, and 

 which are commonly known under 

 the name of ' silk-weeds,' or ' crow- 

 silk.' Their form is usually very 

 regular, each thread being a long- 

 cylinder made up by the union of a 

 single filament of short cylindrical 

 cells united to each other by their 

 flattened extremities ; sometimes 

 these threads give off lateral 

 branches, which have the same 

 structure. The endochrome, though 

 usually green, is occasionally of a 

 brown or purple hue, -and is usually 

 distributed uniformly throughout the 

 cell (as in fig. 430). The plants of 

 this family are extremely favourable 

 subjects for the study of the method 

 of cell-multiplication by binary sub- 

 division. This process usually, but 

 not always, takes place only in the 

 terminal cell ; and it may be almost 

 always observed there in some one of 

 its stages. The first step is seen to be 

 endochrome, and the inflexion of the 

 (fig. 430 A, ) ; and thus there is 

 hour-glass contraction across the cavity 



FIG. 430. Process of cell-multipli- 

 cation in Claclophora glomerata'. 

 A, portion of filament with incom- 

 plete separation at a, and complete 

 partition at b ; B, the separation 

 completed, a new cellulose parti- 

 tion being formed at a ; C, forma- 

 tion of additionallayers of cellulose 

 wall, <, beneath the mucous in- 

 vestment, (J y and around the 

 ectoplasm, , which encloses the 

 endochrome, l>. 



gradually 



the subdivision of the 

 ectoplasm around it 

 formed a sort of 

 of the parent-cell, by 



which it is divided into two equal halves (B). The two surfaces 

 of the infolded utricle produce a doable layer of cellulose mem- 

 brane between them. Sometimes, however, as in Cladophora 

 glomerata (a common species), new cells may originate as branches 

 from any part of the surface by a process of budding, which, 

 notwithstanding its difference of mode, agrees with that just 

 described in its essential character, being the result of the sub- 



