AQUATIC PLANTS USEFUL TO THE MICROSCOPIST. 57 



jectj within certain limits, the better. Many beginners 

 make the mistake of trying to examine too large a speci- 

 men or too much of a mass at once. 



LEMNA POLYRRHfZA (Fig. 12) AND LEMNA MfNOR (Fig. 13).- 

 DUCKMEAT. 



These are small plants, very common, and often so 

 abundant that the entire surface of large ponds is cov- 

 ered by them as by a green carpet. The water in such 

 cases is so completely covered and concealed that the 

 observer is for a moment tempted to step on it. The 

 above two species resemble each other, yet they differ 

 so widely that a glance will distinguish them. Each 

 consists of a small green, more or less oval leaf or frond 

 floating on the water, with one or more rootlets hanging 

 from beneath, but never taking root in the 

 mud. Usually two, three, or four fronds 

 are attached together, so as to form an ir- 

 regular star. Lemna poly rrhiza, the many- 

 rooted one (Fig. 12), has the largest fronds, 

 is a deeper green, and, as its specific name Fig. 12. Lemna 

 signifies, has many rootlets, often a dozen, 

 hanging in a cluster from each. It can always be 

 known by this root-cluster and by the dull purple color 

 of the lower surface. It seems to like the sun better 

 than Lemna minor, and is oftener found abundantly on 

 open ponds, while the latter appears to prefer ditches 

 with high banks and shade. 



Lemna minor (Fig. 13) has smaller, more oval and 



