488 TRANSACTIONS OF THE 



are spent in building up vegetation, and that wlien all tlie con- 

 ditions of vegetation are present, such as the germ, water enougjb 

 and a proper soil, the resulting vegetation is the annual exponent 

 of his accumulated rays. 



Bousingault has shown that the aggregate action of the sun for 

 building up plants is about the same, and that the different va- 

 rieties are provisions for availing of different climates. Thus 

 barley growing in 



Egypt requires 90 days at a mean temperature of 67 deg. 48 min, 

 Tuguleres, do 168 do do 50 " 12 "^ 



Santa Te De Bogota, requires 122 days at a mean 



temperature of, 57 '- 24 "^ 



The same results were obtained for wheat, maize, and the 

 potato. It is probably for this reason that winter wheat, requir- 

 ing more of the sun's forces for its growth, has more value thaci 

 spring wheat. 



Thus the mass of vegetation gradually decreases from the 

 tropics towards the poles, and such plants as maize, whose varie- 

 ties adapt them to extremes of latitude, give the greatest annual 

 product where they assimilate the most of the rays of the sun. 

 The apparent exceptions to this rule in the ca^e of deserts, do but 

 confirm it in reality, for wherever water occurs for any cause,, the 

 most arid plain is immediately covered with vegetation. In 

 Inquiring into the causes of the success of irrigation lately, we 

 found that the annual fall of rain in tropical regions was greater 

 — being on an average 100 inches annually — while in oiu' own 

 climate it is only about 35 inches, and ceases as we proceed north 

 with the region of trees. We found, also, that in most countries 

 the action of the sun on the j)iant was, so to speak, able to digesi 

 more rain than falls, and hence the advantages of an artificial 

 supply. That the perfection of irrigation in any country would, 

 be to deprive it of all its rivers, except in cases where, for topo- 

 graphical reasons, the fall of rain is beyond a certain limit, a re- 

 sult which the extensive arrangements for irrigating the plains 

 about Babylon and Nineveh must have in great measure effected. 



Connected with this matter of the great amount of rain in tro- 

 pical climates, it is interesting to notice that the water upon the 



