252 BOTANICAL GAZETTE [OCTOBER 
fore, that during the driest months of the year the majority of the 
- plants of the hill require an amount of soil moisture of 8-10 per cent. 
With a drier soil than this it is probable that most forms without 
pronounced storage organs would succumb in a few weeks. 
No determination of wilting points is as yet available for the 
other three soils, but from the well-known fact that the ‘‘non-avail- 
able water”’ in soils decreases, in general, with their water-holding 
power, it is probably safe to assume that vegetation can withstand 
a considerably drier soil in the case of the slope and wash, and a 
somewhat drier one in the case of the river plain. 
From the graphs it is evident that the period of most intense 
drought occurs just before the beginning of the summer rains. In 
general, the month of June may be taken as the month of driest 
soils, and it is also the month of greatest evaporation. There- 
fore, for plants without well-developed water storage tissues, this 
month must approximate the critical period of the year so far as 
the water relation is concerned. It will therefore be instructive to 
derive the average soil moisture content for this month in the case 
of each of the four soil types. So far as Iam aware, the root systems 
of all plants of this area, which are without storage tissues and which 
are not annuals, penetrate into the soil at least to the greater of 
the two depths here considered. We may therefore consider the 
greater depth alone. The following table presents these averages 
for the four determinations taken from June 2 to July 2, 1908. 
The water-holding capacities are also given, and the water content 
as percentage of the latter. 
SOIL MOISTURE AT THE CRITICAL PERIOD 
( 
AVERAGE MOISTURE CONTENT, JUNE 1908 
WATER-HOLDING 
Som TYPE POWER, p tage of 
IN PERCENTAGE ercentage, 
erie iliac Depth in cm. geet : ge 
| 
be 6 Cale 6 Cea a acme 48 30 Io 9 22. 7 
Slobé... 25s Gans 20 20 4:5 : ze 
ash ek oe eee 25 30 2.1 pais 
Plain aca eta ea Sk 39 30 6 3 [oS e 
That the vegetation of the hill is the most varied and the er : 
perennially active of all the plant societies of our area, agrees W° 
