232 METHODS IN PLANT HISTOLOGY 



stones in quiet or running water. The mats feel rough and crisp and, 

 even under a pocket lens, show the characteristic branching by which 

 the form is easily recognized. The absence of a mucous coat makes 

 Cladophora a convenient host for numerous parasitic algae, among 

 which diatoms belonging to the genera Cocconeis and Gomphonema 

 are particularly abundant. 



For laboratory cultures, select the forms found in quiet water, but 

 for preparations, forms growing where the waves dash hard are better, 

 since you can get a fine display of branches under a small cover. Forms 

 growing in still water or in gently flowing water may look like un- 

 branched filaments, under an ordinary cover. 



For fixing, use chromo-acetic-osmic acid, watching the effect of the 

 solution and modifying the constituents until you find just what you 

 need for that lot of material. If it looks all right at the end of 10 min- 

 utes, it is likely to remain all right. Cladophora is one of the most 

 difficult of all algae to fix well. Iron-alum haematoxylin, followed by 

 the Venetian turpentine methods, gives the best results for nuclei and 

 pyrenoids. Phloxine and anilin blue are better for the cell wall and 

 chromatophores (Fig. 54). 



Ulothrix. — Where the problem of the origin and evolution of sex is 

 studied, Ulothrix is an indispensable type (Fig. 55). Ulothrix zonata is 

 found in springs, brooks, and rivers, occurring in bright green masses 

 attached to stones in riffles, especially in sunny places. It is abundant 

 on stones and piles along the beaches of lakes. Nuclear division takes 

 place at night, most abundantly about midnight, and is followed by a 

 rapid development of zoospores and gametes, which continue to be 

 discharged throughout the forenoon. In the afternoon the material is 

 largely vegetative. Another species is found in stagnant ponds, 

 ditches, and even in watering-troughs and rain-barrels. It is difficult 

 to keep in the laboratory the forms which are found in rapidly flowing 

 water. However, if they are brought in still attached to stones and 

 placed under a stream of tap water, they may live for a couple of 

 weeks and may produce zoospores every morning. The production of 

 zoospores may continue for a few days, if the material is merely put 

 into a jar of water; in a 2-4 per cent cane-sugar solution the produc- 

 tion of zoospores continues a little longer. 



No form is better than Ulothrix for illustrating to a class the differ- 

 ence between zoospores and gametes. Even when gametes are not con- 

 jugating, their more rapid movement is noticeable; and when conju- 



