— 161 — 
with RAunkıer’s statement that the Therophytes are the 
growth-form best adapted to a dry hot climate, but that they 
decrease in numbers where the temperature is low. 
As already stated the most important characteristic of 
the biological spectrum of the Transcaspian lowlands is the 
great number of Therophytes or annual plants (41 per ct.). 
This is also characteristic of countries to the south and 
south-west, and in the desert areas of western North-America. 
(see table 3 p. 159). According to Mac Doucaz (1909) and 
CANNON (1909); tlie:Sonora desert has two precipitation-maxima 
with spells of dry weather between (comp. p. 59); duri -g the 
two wet periods there is a great abundance of mesophytic 
annual plants (ephe~erals), the species of the two periods 
being different althi: A’ some of them appear twice year 
both in summer and w. er. (See also THORNBER |. c.) 
Most of the annual plants of Transcaspia belong to 
what VOLKENS (l. c. p. 20) terms ephemerals, that is plants 
which complete their life-history from germination to the 
ripening of seeds during the short and comparatively moist 
spring-time (see above p. 59). In countries where the winter 
is warm, germination sometimes takes place during autumn 
or winter (VOLKENS p. 19), but in the cold winter of Trans- 
caspia this is not likely to be the case. 
The existence of the ephemeral plants is dependent on 
a short comparatively moist favourable period, sufficiently 
warm and long enough to permit them to complete their 
development. The adaptations of the plants consist in the 
capacity for rapid development and in the resistance of the 
seeds to prolonged dessication; their vegetative parts on the 
other hand show only slight adaptation against dessication, 
so that they must be termed mesophytic in structure. 
An important condition for the welfare of the ephemeral 
plants, which also holds good as regards the longer lived 
annual plants, is that sufficient open ground is available and 
that the perennials or the ligneous species do not grow so 
close as to hamper the germination and growth of those 
plants which must start afresh from seeds every year. This 
condition is fulfilled in the Transcaspian lowlands where a 
dense vegetation is only found in the valleys of the rivers and 
11 
