—68— 



half years) of continuous drying, some of the leaf cells of Grimma apocarpa stilF 

 remained alive. 



3. Furthermore, the drouth-resistance within the species fluctuates accord- 

 ing to the water-content of the habitat, becoming adjusted to this water-con- 

 tent through the adaptability of the protoplasm. Thus Bryum argenteum, 

 when cultivated in damp air resisted 8 weeks of drying, while the same species 

 cultivated under submerged conditions succumbed in 6 weeks. Still more 

 instructive, however, is the case of species that have grown naturally in dif- 

 ferent habitats. A striking example of this is seen in Hypnum aduncum, where 

 it was found that specimens of a terrestrial form remained alive in part after 

 20 weeks of drying, while specimens of an aquatic form were killed after 4 weeks. 



4. On account of the habit of growth of many mosses in cushions or thick 

 mats, a reduction of the evaporating surface is attained and a consequent re- 

 tardation of the loss of water during times of drouth. Thus after 18 weeks of 

 drying Ceratodon purpureus remained alive if in mats, but was killed if the 

 shoots had been separated. 



5. In contrast to a continuous dry period, alternate drying and wetting 

 injures the moss protoplast relatively quickly. For instance, Bryum capillare 

 was killed after 10 such alternations. 



6. The protonema appears to have a power of drouth-resistance similar in 

 character and amount to that of the leaf cells. 



7. The great importance of the calyptra in protecting the young sporogo- 

 nium is shown by the following comparison: 



with calyptra without calyptra 



Dicranum fuscescens 16 weeks, alive 5 weeks, dead. 



Mnium hormim-. 10 weeks, alive 4 weeks, dead. 



Barbula muralis 25 weeks, alive 6 weeks, dead. 



Funaria hygrometrica 10 weeks, alive 2 weeks, dead. 



8. The determination of the minimum temperature for moss shoots show& 

 that most species resist a temperature of 10° C.(+i4° F.) without evident injury, 

 and that the freezing point for the leaves of most species is above — 20° C. ( — 4° 

 F.). At — 30° C. ( — 22° F.) the most resistant species were killed. 



9. The freezing point varies according to the temperature prevailing at 

 the time of growth, whence we may conclude that there is a direct adaptability 

 of the vegetative structure to temperature. Funaria hygrometrica, when cul- 

 tivated at +20° C. was partially killed at — 5° C; while material collected out 

 of doors in January was not completely killed even by a temperature of — 15° C. 



10. The growing points of the moss shoots and the basal cells of the leaves 

 are far superior to the ordinary leaf cells in drouth-resistance and in cold-re- 

 sistance, and through their regenerative power, after the new intake of wacer 

 or the return of higher temperatures, form a new system of shoots or protonemata 

 making possible a renewed existence of the individual moss plant. 



11. It is shown that there is a relation between the strength of the cell sap 



