DESMIDS, DIATOMS, AND FRESH-WATER ALGjE. 69 



/'. Cell-wall smooth, rough, warty, or spine-bearing, 

 also soft and flexible; always floating freely, 

 never growing on stems permanently adherent 

 to other objects, but sometimes attached side by 

 side to form long bands or ribbons; a vacuole 

 with swarming granules often present in each 

 end. {Dismids', I.) 



c. Cell-wall marked transversely, often -also longi- 



tudinally, by lines, smooth bands, or dots; 

 never spine-bearing; cell-wall hard and brittle; 

 floating freely, or growing on colorless stems 

 permanently attached to other objects. {Dia- 

 toms, II. ) 



d. Plants forming cloud-like clusters, long stream- 



ers, or scum-like floating masses visible to the 

 naked eye; color bright green or olive, some- 

 times almost black; the cells under the micro- 

 scope united end to end to form long, sometimes 

 branching filaments. {Alga, III.) 



I. DESMIDS. 



As the desmids are singly invisible to the naked eye, 

 the student can know what he has gathered only after 

 reaching home, except in those rare instances where 

 the little plants have become congregated together in 

 such quantities that a good pocket-lens will show their 

 forms. I have more than once found Closterium in this 

 profusion, but never any other. The early spring, as 

 early as the last of March or the first of April, in the 

 writer's locality (central New Jersey), is perhaps the 

 best time of the year to gather them, or indeed any of 

 the Algge. At that time all these plants seem more 

 vigorous, their vital functions are performed more 



