DEVELOPMENT AND REPRODUCTION 



333 



more by certain substances than by others; fern sperms are strongly attracted 

 by malic acid and still more attracted by the common soluble salts of this acid, 

 while moss sperms are most attracted by cane sugar. There appears to be no 

 doubt that the maturing moss archegonia secrete a special substance that 

 attracts sperms of the same species. Upon the sperms of other plants this 

 substance appears to have no effect. 



The reproduction of fungi is also influenced by a large number of external 

 conditions. 1 It is generally true that reproduction does not occur in algae 

 and fungi under conditions favorable to vegetative growth, while conditions 

 favoring reproducton usually retard vegetative growth. 2 



Sexual consanguinity 6 is necessary for the union of the sexual cells of seed- 



Fig. 171. — Germinating pollen-grains of Vallota purpurea, their tubes directed toward a mass 



of diastase. (After Lidforss.) 



plants as well as of spore-plants. The chemotaxis of sperms (as of ferns) is 

 paralleled by the chemotropism of the pollen-tubes of flowering plants. Just as 

 the sperms swim toward the source of diffusion of the attracting substance (such 

 as malic acid), so do the pollen-tubes bend and elongate toward this source. 

 Fig. 171 shows a culture of pollen-tubes of Vallota purpurea growing in a 30-per 



1 Klebs, Georg, Zur Physiologie der Fortpflanzung einiger Pilze. III. Allgemeine Betrachtungen. 

 Jahrb. wiss. Bot. 35: 80-203. iooo. 



2 Also, see: Jickeli, Karl, F., Die Unvollkommenheit des Stoffwechsels als Veranlassung fur Vermehrung, 

 Wachsthum, Differenzierung, Ruckbildung und Tod der Lebewesen im Kampf urns Dasein. Berlin. 1902. 



b This and the next following paragraph are added from the 7th Russian edition. — Ed. 



