CLASSIFICATION OF DESMIDIACE.E. 2G0 



in the marked difference in the proportions of its cells ; such elon- 

 gated forms as Closterium (Figs. Ill, 114), in which the length of 

 the frond is many times its breadth, being thus separated from 

 those in which, as in MicraMerias (Fig. 112), Cosmarlum (Fig. 

 113), and Staurastrum (Fig. 110), the breadth of the frond more 

 nearly equals the length. In the former the Sporangia are smooth, 

 whilst in the latter they are very commonly spinous and are some- 

 times quadrate. In this group the chief secondary characters are 

 derived from the degree of constriction between the two halves of 

 the frond, the division of its margin into segments by incisions 

 more or less deep, and its extension into teeth or spines. 



208. The Desmidiacece 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, especially in open exposed situations, 

 such as boggy moors, are most productive. The larger and heavier 

 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 water, 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 by the straining of repeated quantities, 

 a considerable accumulation may be gradually made. This should 

 be then scraped off with a knife, and transferred into bottles with 

 fresh water. If what has been brought up by hand be richly 

 charged with these forms, it should be at once deposited in a 

 bottle ; this at first seems only to contain foul water ; but by 

 allowing it to remain undisturbed for a little time, the Desmi- 

 diacece will sink to the bottom, and most of the water may then be 

 poured-off, to be replaced by a fresh supply. If the bottles be 

 freely exposed to solar light, these little plants will flourish, 

 apparently as well as in their native pools ; and their various 

 phases of multiplication and reproduction may be observed during 

 successive months or even years. — If the pools be too deep for the 

 use of the hand and the scoop, a Collecting-Bottle attached to a 

 stick (§ 176) may be employed in its stead. The Ring-Net (§ 176) 

 may also be advantageously employed, especially if it be so con- 

 structed as to allow of the ready substitution of one piece of 

 muslin for another. For by using several pieces of previously 

 wetted muslin in succession, a large number of these minute 



