410 BOTANICAL GAZETTE [NOVEMBER 
No thermograph records are available for subterranean tempera- 
tures in the Hawaiian lava flows, but such will very likely correspond 
closely with the results obtained by Cannon.” In his study of 
the root relations of desert plants at Tucson, an almost continuous 
record was made of the soil temperatures at a depth of 15 cm. for 
the 5 years 1905-1909. CANNON states: 
The record shows an undulating record of which the curve crests cor- 
respond to the warmest for each day, and the depressions the coldest. The 
crests . . . . are remarkably uniform in height, as also the depressions are 
uniform in depth. The difference between the crests and the depressions is 
about 8° F., with 12° as the greatest variation. .... Owing to the lagging 
of the soil temperatures, as compared with those of the air, the maximum is not 
attained until about 6 P.M., and the minimum about midnight. 
A careful quantitative and qualitative investigation of the root 
relations of the lava inhabiting species is yet to be made, but it 
_ already gives promise of yielding some valuable contributions to 
our knowledge of plant ecology. To quote again from MaAc- 
Doveat (loc. cit., p. 82): 
It may be said, in conclusion, that the facts disclosed as to the actual 
temperatures in the soil, the diurnal and seasonal change therein, lead to the 
belief that the differences in temperature of the aerial and underground portions 
of plants cannot fail to be of very great importance in the physical and chemical 
processes upon which growth, cell-division, nutrition, and propagation depend. 
The determination of the effect of differences in temperature between the roots 
and aerial shoots has received but little consideration from the physiologist 
and the geographer. 
Plant invasion on lava flows 
The rate and amount of invasion is chiefly dependent upon two 
factors: (1) proximity of adjacent vegetated regions from which 
invasion may take place; (2) amount of precipitation, determining 
the character and abundance of invading forms. A lava flow which 
cuts a path through the humid jungle forest is soon (30-50 years) 
disintegrated and overgrown. A lava flow on an arid summit 
slope (8000-10,000 ft.) will remain practically naked for centuries. 
Between these two extreme types every intermediate stage can be 
found (figs. 11, 19, 20). 
2 Cannon, W. A., Root habits of desert plants, p. 20. 
