These factors are of four kinds ; external and internal, posi- 

 tive and neg^ative. The external factors are relative humidity, 

 temperature, Avind movement, and soil, and are arranged in the 

 order of value. The internal factors are plant structure, or- 

 gans, vitality, and shape and size of the plant. Each of a;l 

 these factors may be positive or negative, that is, favorable 

 or unfavorable to' the life of the plant. It goes without saying 

 that a moist and warm chmate with a rich, loamy soil will be 

 far more productive than a dry and cold climate with a clay 

 soil. Even a novice would hardly expect to find the same vege- 



leys. There are other facts, however, that are not so well 

 known, for geology shows that our climate has been subjected 

 to great changes so that the perpetual summer of the Tropics 

 has disappeared, and with it have gone the elephants, tigers, 

 Tropical birds and plants which once were here. To the stu- 

 dent of Nature it is an axiom that vegetable life must be m 

 harmony with its surroundings, and that as the climate changes 

 the species must change, move out or die. So he expects to 

 find evidence of the hand of Time in the life of a region, and is 

 not surprised to see the birds and deer migrate, the bear hole 

 up, and the squirrels burrow and store food, while in the plants 

 which can neither burrow, hole up nor migrate he counts_ on 

 finding devices for protecting themselves against destruction, 

 and for propagation and distribution 



favorable we find a region of dense and luxuriant vegetation 

 whose only negative factor is a struggle for life between 

 species, a struggle for light, air and soil, this results in a slen- 

 «der habit, and an abnormal number of perennials, climbers, 

 trees, parasites and epiphytes. In the dry Tropics the absence 

 of humidity turns almost every other factor into a curse and 

 so the struggle of species ceases and every device of Nature is 

 used to keep life in the plant. This produces thorny, starved, 

 J^tunted and fleshy perennials, and multitudes of annuals, but 

 these are not able to cover the nakedness of the drifting sands 

 cr crinkling deserts. 



