NATURAL HISTORY OF BHOMPSTON POND. 225 



minutes. Oil bodies and pyrenoids (d) can be detected in the 

 chloroplast. The conjugation of two cells can sometimes be 

 observed ; it is accomplished by the approximation of the 

 convex sides of two bent cells. The wall between is then 

 absorbed, and the protoplasmic contents of the two cells fuses 

 and a zygospore is formed between the two filaments. Another 

 member of this order, Spirogyra (fig. 87), is composed of 

 a single row of cells, each of which contains a spiral band- 

 like chloroplast (a), studded with pyrenoids (b) encircled 

 by starch granules. (The later are best demonstrated by 

 irrigating a little iodine.) The nucleus (c) is suspended 

 between the vacuoles (d) by protoplasmic strands from the 

 primordial utricle (e). Conjugating filaments (fig. 88) are 

 often found. Two opposite cells are each seen to develop a 

 process both of which finally meet and fuse (scalariform), at 

 the same time the cell contents of each become rounded up. 

 The contents of one (the male) then pass over and fuse with 

 the contents of the other (the female), forming a zygospore (a), 

 which then acquires a thick wall to resist cold and drought. 

 An allied plant, Zygnema (fig. 89), is another pretty little 

 filamentous form. It chiefly differs from Spirogyra in having 

 two stellate chromatophores (a) with the nucleus (b) between. 

 It is found growing in association with Spirogyra, &c. 



The Diatomese occur in large numbers, partly on the stems 

 of the aquatic plants, but perhaps more profusely in the 

 bottom, where they move along very slowly by means of 

 protoplasmic processes. The cell-wall is of extreme interest, 

 not only on account of the great diverstiy and fineness of its 

 markings, but also because of its great durability. The cell 

 wall consists of two valves, which fit together like a box and 

 its lid ; it contains so much silica that neither heat nor acid 

 will destroy its shape or markings. The chromatophores 

 are brownish and there is a single central nucleus. Pinnularia 

 oblonga (fig. 90) and many other species are to be found. 



Pandorina (fig. 91), a small colonial form, consisting of 

 sixteen bi-ciliated protoplasts (a) embedded in a colourless 

 gelatinous matrix (b). The colony swims by the combined 



