MICROSCOPIC FORMS OF VEGETABLE LIFE. 



269 



Ens. 



of the filamentous group, in which the zygospores are always globular 

 and smooth, is based on the fact that in one set of genera the joints are 

 many times longer than they are broad, and that they are neither 

 constricted nor furnished with lateral teeth or projections; whilst in the 

 other set (of which Didymoprium, Fig. 160, i's an example) the length 

 and breadth of each joint are nearly equal, and the joints are more or 

 less constricted, or have lateral teeth or projecting angles, or are other- 

 wise figured; and it is for the most part upon the variations in these last 

 particulars, that the generic characters are based. The solitary group 

 presents a similar basis for primary 

 division in the marked difference 

 in the proportions of its cells; such 

 elongated forms as Closterium 

 (Figs. 156, 159), in which the 

 length is many times the breadth, 

 being thus separated from those in 

 which, as in Micrasterias (Fig. 

 157), Cosmarium (Fig. 158), and 

 Staurastrum (Fig. 155), the 

 breadth more nearly equals the 

 length. In the former the sporan- 

 gia are smooth, whilst in the latter 

 they are very commonly spinous and 

 are sometimes quadrate. In this 

 group, the chief secondary charac- 

 ters are derived from the degree of 

 constriction between the two halves 

 of the cell, the division of its mar- 

 gin into segments by incisions more 

 or less deep, and its extension into 

 teeth or spines. 



269. The Desmidiacew are not 

 found in running streams, unless 

 the motion of the water be very 

 slow; but are to be looked-for in 

 standing though not stagnant 

 waters. Small shallow pools that 

 do not dry up in summer, especi- 

 ally in Open exposed situations, binary subdivision and Conjugation of Didy- 

 J , ' moprium Grevillii:, portion of filament, sur- 



SUCll as boggy mOOrS, are most rounded by gelatinous ebvelope; B, dividing cell; 

 TYiWlnnfi-c-a Tl-in Invrror *r\(\ TIPMV c> single cell viewed transversely; D, two cells in 



pi oductiye. I lie iai ger ana nea\ - c ^ nju | ation . K> formation of lygoipore. 

 ler species commonly lie at the 



bottom of the pools, either spread-out as a thin gelatinous stratum, or 

 collected into finger-like tufts. By gently passing the fingers beneath 

 these, they may be caused to rise towards the surface of the waters, and 

 may then be lifted out by a tin-box or scoop. Other species form a 

 greenish or dirty cloud upon the stems and leaves of other aquatic 

 plants; and these also are best detached by passing the hand beneath 

 them, and ' stripping' the plant between the fingers, so as to carry off 

 upon them what adhered to it. If, on the other hand, the bodies of 

 which we are in search should be much diffused through the water, there 

 is no other course than to take it up in large quantities by the box or 

 scoop, and to separate them by straining through a piece of linen. At 

 first, nothing appears on the linen but a mere stain or a little dirt; but 



