160 THE GEOGRAPHICAL DISTRIBUTION 



same plane, when the stem becomes horizontal or its 

 branches take a drooping growth. This change of position 

 may be observed especially in the ground-ivy (G-lechoma 

 hederacea.) The direction of the branches of trees is 

 also greatly influenced by the light. Thus the lower 

 branches are more horizontal and stretch further from the 

 stem than those towards the summit of the tree. 



Humidity. — Vegetation is greatly promoted by a moist 

 condition of the atmosphere. Water is as necessary in 

 germination, as in all the other phenomena of vegetable 

 life. It penetrates into the substance of the seed, softens 

 its envelopes, and makes the embryo swell. It therefore 

 places the seed in the conditions which are the most favor- 

 able for germination. 



The quantity of rain which falls, varies greatly in dif- 

 ferent parts of the world. There are enormous tracts of 

 land on which rain never falls. In some places it rains 

 almost perpetually. Between the tropics, the rains follow 

 the sun ; when he is north of the equator, the rains prevail 

 in the northern tropics, and when he is south of that line, 

 in the southern tropics ; hence one half of the year is ex- 

 tremely wet, and the other half very dry. The changes 

 take place at the equinoxes, when the sun crosses the line. 

 In the temperate zone rain falls at all seasons, though 

 more abundantly in some, than in others. This unequal dis- 

 tribution of heat, light, and rain, over the earth's surface, 

 must necessarily produce a great difference in vegetation. 



If we would however approximate to more correct 

 and philosophical views respecting the influences of these 

 grand stimulants of vegetation and animality, it is neces- 

 sary to consider their influence, not so much separately, as 

 in a state of combination, and its efi'ect in different parts 

 of the earth. 



