sspears 
THE CONDITIONS FOR THE DISTRIBUTION OF PLANTS j05 
the wild mango, or are strengthened by rooting pillars or secondary 
stems, formed by aérial roots, as the Banyan, Pandanus and Rhizo- 
phora. By all these structures the individual strength of the tree is 
enhanced and its resisting power increased, while other trees are so 
constructedas to reduce the extent of the resisting surface. Ina few 
Indian trees, as in Hriodendron anfractuosum, one of our silk-cotton 
trees, and in Termindlia catappa, the “ Badam,” all the branches are 
horizontally arranged in regular whorls, between which there is 
ample room for the wind to pass. Similar structures are found m a 
great many Coniferce, noticeably in Araucarias, where also the small 
linear or scaly leaves tend to expose a very small surface to the ac- 
tion of the wind. Such a limited development of the leaves is a com- 
mon feature in many plants, as in most Australian Leguminous plants, 
in Casuarinas, Tamarix and many others. Very finely divided, 
pinnate or bipinnate leaves, as in Poinciana, Mimosa, Albizzia, Sc., do 
perhaps render similar services to the plants, which seems to be the 
more necessary as the wood of such trees is frequently very brittle. 
The next and last of climatic agencies of which ¥ shall speak are 
rain, humidity and moisture. Of these the local annual rainfall is 
chiefly important for distribution of plants, in fixing certain hmits, 
but also humidity of the atmosphere and moisture of the ground are 
important items. A certain quantity of each of these is necessary 
for all plants; but it is and will always be extremely difficult to 
ascertain the exact requirements of the different species, and we can 
only by experience learn that one requires next to no supply of 
water or damp air, while another requires to be constantly soaked. 
How plants are protected against a minimum of rain or moisture 
has been mentioned in connection with their protection against heat, 
and it only remains to be seen how they are guarded against ex- 
cesses. In this respect may be mentioned local shelter, woody 
structure, bark, cuticula of the leaves, secretions of waa and fatty 
substances and a layer of condensed air, that frequently surrounds 
the leaves of many plants and prevents their wetting, as in the fronds 
of Maidenhair-ferus, which when dipped in water look as if they 
were silvered and remain perfectly dry. The parts of plants most 
susceptible to the effects of “ain and moisture are the flowers or at 
least the soxual organs, which are in most cases more or less pro- 
tected. Their means of protection are however too numerous to 
be recorded here, and I shall only mention a very few, as the fre= 
quent nodding position of the flower, the closing of the petals at night 
