314 BOTANICAL GAZETTE [ocroBER 
heterothallic, contains a large majority of the species (ex. Rhizopus, Mucor, 
Phycomyces), each of which is made up of two sexual strains, so that the “zyg0- 
spores are developed from branches which necessarily. belong to thalli or mycelia 
diverse in character and can never be obtained from the sowing of a single spore. 
ee very heterothallic species is, therefore, an aggregate of two distinct strains, 
through the interaction of which zygospore production is brought about.” 
These sexual strains show in general a greater or less vegetative luxuriance 
and are designated by the + and — signs respectively. The two strains 
form zygospores when growing together, as the progametes “arise from the stimt- 
lus of contact between the more or less differentiated hyphae (zygophores) and 
are from the outset always normally adherent.” 
“A process of imperfect hybridization will occur between unlike strains of 
different heterothallic species in the same or even in different genera,” é. ¢., the 
gametes are formed by the chemotactic stimulus of contact with the mycelium 
of an opposite strain. This peculiarity makes it possible to determine the strait 
of an unknown form by cultivation with the strains of determined material and 
is most interesting as evidence that the stimulus to zygospore formation is cheat 
ical rather than the rougher physical conditions. These attempts at hybridization 
were not observed to go farther than the cutting off of the two gametes. 
BLAKESLEE concludes from his studies: (a) that the formation of SSE 
is a sexual process; (b) that the mycelium of a homothallic species 1s bisexual; 
(c) that the mycelium of a heterothallic species is unisexual; and further (d) that 
in the + and — series of the heterothallic group the two sexes are represented — 
B. M. Davis. 
THE CONDITIONS influencing the production of zoospores in Chea 
have been studied by FRANK,35 who shows that a decrease in concentration 
Knop’s solution acts as a stimulus, as does also, but in a secondary way, — 
he pana ie stad clusion that 
in light intensity. Temperature limits were also studied, with the con ae 
this factor is only a secondary one in the production of zoospores: a 
bears concentrations up to 2.5 per cent. Knop’s solution. In the ge a 
centrations the cells are larger and their contents more dense. On wes J 
strata soaked with solution the plant behaves much as in a more — 
solution. The transfer of cells from Knop’s solution to solute vty scoot 
single chemical salts influences the production of zoospores vari Be re poi 
ing to the salt used. Thus, as has been shown before, K is so mee d wherein 
sonous than Na. With all the salts used a concentration is soon spe: panes 
no zoospores are produced. The osmotic pressure of the solution at . equal 10 
NaS ; times 1S 4 
tration-limit sometimes lies above, sometimes below, and some ni 
that of the limiting concentration for Knop’s solution. From | a 
concludes that the stimulus producing zoospores is not an osmotic 
: é : ‘ te neal & 
mere reduction of concentration in the nutrient medium 1s INV 
der Chiam) 
: F : : : 
35 Frank, THEopor, Cultur und chemische Reizerscheinunge 
domonas tingens. Bot. Zeit. 621: 153-188. pl. 6. 1904- 
