GROWTH, ETC., I\ THE DESMIDIAGE^. 285 



The rule of growth seems to be the establishment and main- 

 tenance of an equilibrium of life between the semi-cells of every 

 Desmid. If external circumstances upset this equilibrium (as in 

 division) the whole energy of the cell is put forth to restore it again 

 by the development of the new semi- cell to an equaliiy with the 

 other, but jig more. However immature the original semi- cell may 

 be, it is impossible for the nascent semi-cell to surpass it, because 

 there is not sufficient force left in the cell after the equilibrium has 

 been established. Further development can take place, but it 

 must be of both semi-cells pari passu. Under ordinary conditions 

 there seems to be a very slow but continuous development of this 

 kind always going on between the periods of cell-division, but for 

 any sudden and considerable growth to take place in the cell as a 

 whole an external stimulus from nature is required. 



Traxsition Forms. — In successive gatherings taken from the 

 same spot at various times careful comparison will often reveal the 

 existence of a series of variations acting as transition forms between 

 several " species." Each form has its own characteristics and 

 size, but the sizes form an ascending series, and there is a general 

 agreement in shape between the variations. Doc. trabecula affords 

 a striking illustration of this. A well-grown form, var. crenulatum 

 {Roy and Bisset) is the most common Desmid of this country, and 

 in the same locality, and even in the same gathering sometimes, 

 an ascending series of stages may easily be traced through var. 

 Ehrenhergii, which is its invariable companion. For instance 

 Doc. trabecula. var. baculoides (R. & B.), lat. bas., 12-16?/ ; var. 

 Ehrenbergii i. minor, lat. bas. 16- 20m, type, 20-25w, f. elongata, 

 25-28;/ : var. Delpontei mihi, lat. bas., 30-39;/ ; var. cremtlatum 

 (R. & B.). lat. bas., 35-54/;. 



All these sizes I have actually observed and measured. With 

 the exception of var. baculoides all the forms are common. Doci- 

 dium may be taken as a representative of the long Desmids, in 

 which it is particularly differences of length and breadth, specially 

 the latter, which mark ofi the stages of a species one from another. 

 Under such circumstances, of course, transition forms are such as 

 are found to bridge the gaps between those of one diameter and 

 those of another. In Cosmarium and other closely related genera 

 where outline and ornament are the distinguishing feature transition 

 forms may also be found connecting one form with another and 

 one class of ornamentation with another, showing that these cells, 

 though possessing such characteristic shapes and embellishments 

 as to have been generally considered distinct species, are really 

 growing forms of one species — one shape gradually dissolving into 

 another and one style of ornament into another. (See notes under 

 Cos. rectangulare in Jour. Linn. Soc, N.S.W., 1910, p. 475.) 



Zygospores. — In the elucidation of species and their life 

 histories a great deal of help has been expected from the zygospores, 

 but I do not think that much has yet been obtained. It seems to 

 have been largely taken for granted that every variety of Desmid 



