GROWTH, ETC., IN THE DESMIDIACE.^. 291 



semi- cell the granules were represented by distinct scrobiculae 

 (apparently lacunae). It is evident, therefore, that scrobiculae are 

 interchangeable even with the larger granules. 



Not only so, but a form which has a perfectly smooth mem- 

 brane may develope into one with rows of puncta-granules. Such 

 is St. retusum, Turn., which develops into St. retiistim var. punctu- 

 latum, Eich. and Gutw. (var. granidatum, Borge). Doubt has been 

 expressed with regard to this, but as I observed the two forms 

 together many years ago at Collector, and have lately obtained 

 them again from Fairfield, and carefully examined St. retusum, I 

 am in a position to confirm Turner's observation. 



I desire to make it quite plain that there is no stagnation of 

 life in the Desmidiaceae ; everything develops — the cell, the 

 chloroplasts, the membrane. 



Plankton Forms. — Whilst stagnant water and increased 

 temperature promote cell-division, cooler conditions, rain, or a gentle 

 current of water encourage the development of the cell ifself. 

 The truth of the latter statement becomes quite clear with the 

 observation of forms which have fallen under plankton conditions. 

 Such soon discover latent powers of expansion and ability to 

 proceed to further developments. I do not remember ever to have 

 found St. orbiculare var. germinosnm (a plankton form from the 

 Sydney Water Supply) except under these conditions. As for a 

 free-swimming life, its tendency is to cause great exaggeration of 

 any characteristic feature of outline. Slender forms such as CI. 

 gracile and CI. acutum, as also Synedra acus among the Diatomaceae, 

 are drawn out to an immense length, and that, too, without corre- 

 sponding increase in breadth ; the rays of Staurastrum are more 

 produced and often more divergent in front view, denticulations, 

 spines, verrucae are accentuated, and in the simpler forms spines 

 and processes are often developed, especially at the angles. Micr, 

 Thomasiana var. pulcherrima, G. S. West, AlgcB Yan Yeaii, PI. 4, 

 f. 1, and Eu. quadratum var. perornaium, Playf., ante 1908, PI. xii, 

 f. 1, serve to show how plankton conditions tend to bring out 

 every spine and process and verruca of which a cell is capable and 

 to produce them to an unusual length.^ In speaking of tqese 

 variations as plankton forms, I do not mean that they are separate 

 permanent varieties evolved in the course of ages through remote 

 ancestors having adopted a free-swimming existence, and which 

 are now proper to the plankton. They are simply gi"owth varia- 

 tions produced in ordinary forms by being washed down into 

 a stream or lake and forced to develope under plankton con- 

 ditions. There is no such thing as a plankton species of Desmid. 

 I have consistently used in this paper the word development 

 rather than growth, because the latter might be taken to mean 

 (1) increase in size only, whereas among the Desmids increase in 

 size always means transition into a new shape as well ; or (2) 



1 Cf. W. & G. S. West in Trans. Roy. Soc. Edin., Vol. XLI., part hi, 1905, PI. VI.,figs.l,2, 21 

 and P). VII., figs. 8. 9, 13, 14, 22, for excellent illustrations of this. 



