REPRODUCTION AND DISPERSAL 881 



of reproductive activity, but they influence the character of the reproductive struc- 

 tures ; for example, in some of the rusts the spore walls are thicker in xerophytic 

 situations than elsewhere, and in Bornelina the size, shape, and sculpturing of the 

 spores vary with the culture media and with the illumination. 



In certain marine algae, as Dictyota dichotoma, there is a remarkable periodicity, 

 which seems to be related to external conditions. In England the sex organs de- 

 velop at fortnightly periods, the gametes being liberated at a fixed interval after the 

 highest spring tide. In North Carolina there also is a relation to the tides, but the 

 production of sex organs occurs monthly rather than fortnightly. Similar phenom- 

 ena have been observed at Naples, and in Japan a fortnightly period of gamete 

 liberation has been discovered for Sargassum. The most probable causative stimu- 

 lus of reproductive periodicity is the increased illumination that is associated with 

 the fortnightly recurrence of extreme low water ; at Naples the liberation of gametes 

 appears to be on the day when low water occurs nearest to midday. Factors which 

 modify the tides, such as wind or change of atmospheric pressure, also affect the time 

 of gamete liberation. 



The influence of external factors upon reproductive activity in animals appears 

 to be much less obvious than in plants. However, in Paramoecium and in other 

 infusorians the continuance of favorable nutritive conditions seems to cause con- 

 tmued vegetative^ activity, whereas conjugation is due chiefly to changes in the 

 media. In the water-fleas (Daphnia) there are two kinds of generations, one being 

 composed of males and females, and the other being composed solely of partheno- 

 genetic females. It has been ascertained that parthenogenetic generations result 

 when the conditions for nutrition are favorable, whereas bisexual generations result 

 from conditions unfavorable for nutrition, such as increased concentration of the 

 medium, desiccation, high or low temperature, the accumulation of excreta, and 

 starvation. In nature the bisexual generation is especially to be seen in shallow 

 pools, and in autumn in ordinary ponds. The conditions for the development of 

 parthenogenetic and bisexual generations are very similar in certain other animals, 

 such as rotifers (Hydatina), plant lice (aphids), and the grape-louse (Phylloxera). 

 Under favorable nutrient conditions there may be many successive parthenogenetic 

 generations without any intervening bisexual generations. 



The origin of sexuality. There is little experimental evidence 

 bearing upon the origin of sexuality, although there exist a number of 

 forms with facultative gametes, and even with facultative gamete-produc- 

 ing organs (gametangia). In Ulothrix (figs. 1133, 1134) there are 

 intergradations (e.g. zoospores of intermediate size with two or four cilia) 

 between the large quadriciliate zoospores and the small biciliate gametes, 

 suggesting the possible origin of gametes from zoospores; indeed, it is 

 known that without fusion gametes sometimes develop into plants, quite 

 as do zoospores. In Hydrodictyon similar primordia produce gametes 

 in some media and zoospores in others. In Zygnema stellinum there are 

 intergradations between the isogamous, the heterogamous, and the 

 parthenogeoetic gametes, and in the sea lettuces, Ulva and Entero- 



