OTHER FACTORS INFLUENCING DISTRIBUTION. 33 
TROPICAL REGION. 
This zone embraces in eastern North America a narrow belt along 
the coasts of the southern extremity of peninsular Florida and the 
surrounding keys, and contains part of the West Indian or Antillean 
flora. 
SECONDARY FACTORS INFLUENCING PLANT DISTRIBUTION. 
Besides the primary factors, heat, light, and atmospheric moisture, 
which regulate the distribution of life within the wide areas recog- 
nized as life zones, there are others which exercise a potent influence 
upon the distribution of plants in confining them to narrower bounda- 
ries. The former are due to geographical position, latitude and longi- 
tude, extent of land area, presence and position of mountain chains, and 
prevailing winds and oceanic currents, while the latter are dependent 
on minor local conditions, such as altitude, topography, proximity to 
large bodies of water, all of which influence the lesser fluctuations of 
temperature, precipitation, occurrence of frosts or other extremes, 
secondary currents of the air, and exposure to sunlight. In addition 
to these and of equal importance are the stratigraphical relations deter- 
mining the chemical and physical character of the soil and subsoil, as 
well as the drainage of the surface and the soil beneath, which in itself 
must be considered as one of the most important of the secondary 
factors influencing distribution. 
To this class of factors belong also the facilities possessed by 
different species for dissemination. These consist chiefly of biological 
agencies, special adaptations with which the fruits and seeds of many 
species are provided, to secure their dispersion. This is effected in 
various ways, particularly through the agency of animals. One means 
consists of fleshy, sweetish fruits tempting animals to feed upon them. 
In most instances the seeds are discharged with their vital power unim- 
paired and thus are transferred to distant localities, particularly when 
the fruit has served as food for birds. A second means consists of the 
prickles, hooks, or stiff, hooked hairs covering the surface of the hard 
fruits, by which they attach themselves to wandering animals. 
Distribution of seeds is also effected by atmospheric agency, the 
fruits or seeds being very minute or provided with broad, wing-like, 
or plumose attachments, which enable them to be wafted through 
the air. In some cases the sudden tearing asunder of the parts of 
the fully-matured hard fruit when touched causes the scattering 
of the seeds. Finally, some seeds are disseminated through their 
ability to float about in fresh or saline water without losing the power 
of germination.’ 
'Q, Drude, Handbuch der Pflanzengeographie, 121 (1890). 
15894——3 
