3l6 PLANT GROWTH SUBSTANCES 



motile gametes of opposite sex there is an immediate aggregation of the 

 gametes in cknnps of ten to a hundred or more. This striking reaction, 

 which takes place in less than five seconds, is so clean-cut that there is 

 never any doubt as to whether there is a positive or a negative sexual 

 response. When there has been a positive clumping reaction the fusion 

 of biflagellate gametes can be demonstrated by killing the swarmers and 

 noting the presence of quadriflagellate zygotes. In rare cases (1,15) 

 the clumping is due to many gametes of one sex swarming about a 

 single gamete of opposite sex. In most cases it is thought, and it is 

 definitely known in certain cases (7,18), that there are numerous gametes 

 of both sexes in a clump. 



Up to a certain concentration of gametes the size of the clumps 

 formed after mixing those of opposite sex is correlated with the con- 

 centration, but beyond this concentration there is no further increase in 

 number of gametes in a clump. When mixed in optimum concentration 

 the number of gametes in a clump is not the same from species to species 

 and may consist of two, less than half a dozen, ten to twenty, or a 

 hundred or more. For this reason one can distinguish between different 

 intensities of sexual reaction. 



The first demonstration of excretion of sexual substances from algal 

 gametes was by Jollos (3) and was by means of differences in intensity 

 of sexual reaction in Dasydadiis, a heterothallic marine green alga. Since 

 it was impossible to distinguish between male and female gametes on the 

 basis of size or morphology those of opposite sex were arbitrarily desig- 

 nated as plus and minus. When Jollos mixed plus gametes of low intensity 

 with minus gametes of high intensity he obtained a stronger clumping 

 reaction than that from mixing plus and minus gametes of low intensity. 

 He then placed plus gametes of high intensity in a dish, allowed them to 

 swim there for a couple of hours, and then filtered off the water. Plus 

 gametes of low intensity were placed in the filtrate and kept there for 

 a couple of hours. When these plus gametes were mixed with minus 

 gametes of low intensity the clumping reaction was stronger than that 

 with untreated plus gametes. The same change in reaction was induced 

 in minus gametes of low intensity. Jollos interpreted the increased sexual 

 intensity induced in gametes of low intensity as being due to an absorp- 

 tion of sexual substances excreted into the water by gametes of strong 

 intensity. These experiments also show that both male and female 

 gametes excrete sexual substances. Geitler (2) has also shown that 



