42 COMMON POND SCUM. 



nett. 18 According to Bessey," however, we should 

 consider this case the simplest kind of sexuality, in 

 which there is as yet no differentiation into proper 

 male and female. For the further discussion of sexu- 

 ality in plants, the student is referred to the writings 

 of Pringsheim, 14 Sachs," Ward, 16 Strasburger, 17 and 

 others. 



The two plants previously examined may be found 

 in any month of the year, but the one now under 

 examination dies, and entirely disappears from sight 

 by the time winter has fairly set in. It is reproduced 

 the coming spring by the germination of the zygospores, 

 which lie at the bottom of the water during the 

 winter. These resting spores are admirably fitted for 

 spanning this unfavorable season for vegetation. As a 

 rule they require a long period of rest before reaching 

 the germinating condition, so that while they are 

 formed in the earlier part of the warm season, it is 

 usually not till the following spring that they show a 

 disposition to grow ; they are dense and heavy, and 

 therefore sink to the bottom as soon as set free by the 

 decomposition of the filaments in which they grew ; 

 and lastly, their thick double or triple covering serves 

 as an ample protection to the living protoplasm with- 

 in. 



12 Jour. Linn. Soc., xx (1884), p. 430; Amer. Nat., xvii (1884), p. 421. 



13 Amer. Nat., xix (1885), p. 995. 



14 Monatsber. d. k. Akad. der Wiss. in Berlin, 1869. 

 16 Textbook of Botany, 2nd Eng. ed., p. 986. 



16 Quart. Jour. Micr. Sci., 1884, p. 262. 

 " Op. cit. 



