406 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and a bait" years, and has analysed some 3,000 samples comprising more 

 than 300 species. He records the following facts as to their periodici^jy. 



1. From the point of view of their complete life histories they may 

 be conveniently grouped into winter annuals, spring annuals, summer 

 annuals, autumn annuals, ephemerals, and perennials. Several examples 

 of each are given. 2. The contradictory results of older observers as 

 to spore-production may be due to neglect of the normal periodicity 

 and vegetative age of the algje used. :-;. Some distinct periods may be 

 recognized in the life history of most freshwater algfe : germination, 

 vegetative development, reproduction, dormancy. 4. Most zygospores, 

 oospores, and aplanospores germinate in the spring ; there is, however, 

 germination going on at all times ; and a secondary maximum occurs 

 in the autumn. 5. The factors involved in spore-germination are 

 probably as numerous as those of seed-germination. The importance 

 of temperature has probably been over-estimated. 6. The length of the 

 vegetative period in some forms is quite indefinite. In the Zygnemales 

 and CEdou'oniales it probal)lv has a definite length under normal coudi- 

 tions. 7. Temperature, light intensity, concentration, and mineral 

 content of the water accelerate or retard the approach of the repro- 

 ductive period, s. The normal length of the vegetative cycle in 

 Spirotjyra is an inverse function of the specific surface of the cells. 

 Possibly this is also true of Zygnema and (Edor/omum. 9. The normal 

 length of the vegetative cycle in species of Spirogyra is approximately 

 equal to a constant (sixty-five) divided by the product of the specific 

 surface multiplied by the temperature. 10. The concentrations of the 

 waters in pools, ponds, and surface streams attain their maxima in 

 early spring and autumn, corresponding in general with the periods of 

 heavier rainfall. 11. The lowest concentrations occur in late winter, 

 and at the end of a prolonged drought in summer. 12. The periods of 

 most abundant fruiting of algte correspond with the periods of high- 

 water levels. 13. The concentration of natural waters at their maximum 

 is so small in comparison with the concentrations of the cell sap that it 

 is doubtful whether it is of any significance in initiating reproduction. 

 14. In the Zygnemales, lateral conjugation, scalariform conjugation, 

 and aplanospore production appear to be hereditary tendencies rather 

 than the result of environmental conditions. 



Nuclear Division in Spirogyra bellis.* — M. L. Merriman describes 

 the nuclear division in Spirogyra bellis, and gives the following summary 

 of her results : — 1. Instead of a spireme, as in S. crassa, a disk arises 

 from material condensing within the mass of nuclear plasm and central 

 body. This disk is discernible in both living and stained material, 



2. Xo trace of organization is to be seen in the living disk, but fixed 

 material shows it to arise from aggregations of variable appearance and 

 staining qualities. These aggregations are not the chromosomes. The 

 more deeply stained of these bodies arise from the nuclear plasm, the 

 less deeply stained appear to come from the decomposing central body. 



3. This sphere of aggregated material gradually changes in shape, 

 becoming a cylinder. The more deeply stained masses become arranged 



* Bot. Gaz., Ixi. (1916) pp. 311-24 (3 pis.). 



