ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 319 



the whole district, including the river, but in no case were they fonnd 

 in abundance. Those found in the river were almost entirely different 

 from those found on land. The swamp waters were poor in Diatomacege, 

 the river rich. Two new genera and 17 new species are descril^ed in 

 the paper. 



Extreme Terrestrial Form of Zygnema.* — F. E. Fritsch describes 

 the morphology and ecology of an extreme terrestrial form of Zygnema 

 {Zi/gogonitim) ericetorum (Kiitz.) Hansg., which owes its peculiarities 

 to its very inhospitable habitat on Hindhead Common. The mature 

 cell contains two chloroplasts. After cell-division the daughter-cells 

 contain a single chloroplast for a time. Division is performed by a 

 gradual infolding of the innermost layer of the cell-wall, but is left 

 incomplete for a time, the daughter-protoplasts being still connected 

 through a central pore. Two or three layers are distinguishable in the 

 longitudinal walls. The Hindhead plant remains permanently in the 

 akiuete condition. The outer portion of the wall is strongly thickened 

 and mucilaginous, and plays a great part in protecting the cells during 

 drought and in promoting a rapid recovery afterwards. At the 

 beginning of a drought the fat-globules in the cell move to the 

 periphery and form a very dense layer upon the inner face of the cell- 

 walls. Some hours after moistening the dry plant the fat-globules 

 have become dispersed again. At the beginning of a drought the 

 protoplasts round off slightly and develop a new layer of membrane. 

 Each cell divides at the most but twice between two successive periods 

 of drought. . Another method of akinete formation has been noticed 

 early in the year ; there is an unequal cell-division, resulting in the 

 formation of an akiuete and of a much smaller pigment-cell. The 

 contents of the pigment-cells subsequently disappear, and the empty 

 cells form weak points, where the filament readily ruptures. 



Structure and Origin of Cladophora Balls.f— Elizabeth Acton 

 gives an account of the structure and origin of Cladophora balls, such 

 as are found in certain lakes of Scotland and Ireland, and in other parts 

 of Europe. Papers have been published by F. Brand, C. Wesenberg- 

 Lund and others on the subject. The species which constitute the 

 balls belong to the sub-genus jEgagrojiUa. The balls found in Loch 

 Kildona, S. Uist, consist of C. holsatica Kiitz. They are composed 

 of a densely felted thin living outer-.-phere surrounding a large cavity 

 tilled with plant-debris and mud. The individuals composing the 

 outer-sphere, having lost their apical cells by attrition, emit many lateral 

 branches, some of which have been called " rhizoids " and others 

 "cirrhoids," which act respectively as haptera and clasping tendrils 

 thus Hrmly interlocking the feltwork of plants ; other branches are 

 "stolons" and act as agents of propagation. Multiplication takes 

 place only in vegetative ways, as the main cells slowly die off behind 

 and set the branches free. Sometimes isolated old axial cells rejuvenate 



* Ann. Bot., xxx. (1916) pp. 135-49 (3 figs.). 

 t New Phytologist, xv. (1916) pp. 1-10 (figs.). 



Z 2 



