EFFECTS OF LIGHT, CLIMATE, AND SEASON. 667 



Light is an agent that has a powerful influence on plants, as re- 

 gards their vigour, irritability, secretions, and colour. Hence, in 

 those regions where the light is intense, the vegetation presents cer- 

 tain peculiarities. The luxuriance and greenness of the leaves, the 

 nature of the woody matter deposited, of the fruit produced, and of 

 the secretions formed, are all influenced in some degree by the inten- 

 sity of the sun's rays. Little is known in regard to the effects of in- 

 creased or diminished atmospheric pressure on plants. Humboldt 

 believed that vegetation was influenced by the amount of atmospheric 

 pressure. Further research has not corroborated his suppositions. 

 The effects of the atmosphere have been studied chiefly as regards 

 dryness and moisture, and the mixture of certain gases with it, 

 especially in the vicinity of manufacturing towns (page 159). 



The effects of climate and season on the leafing, flowering, and 

 fruiting of plants, may be seen in the case of some species which are 

 found distributed over various countries in Europe. Berghaus has 

 made an extensive series of observations on the subject. The Lilac 

 (Syringa vulgaris), according to him, unfolds its leaves at Naples, in 

 latitude 41°, during the first half of the month of January; near 

 Paris, in latitude 49°, on the 12th March. The Elder unfolds its leaves 



At Naples . ... January 1-15. 



At Paris .... February 14. 



In England ... . March 8. 



At Upsal . . . March 1-8. 



The Beech unfolds its leaves 



At Naples . . . . End of March. 



In England . . . . 1st May. 



At Upsal ..... Beginning of May. 



In regard to flowering, Berghaus states that in the middle lati- 

 tudes of Europe and North America, it is generally four days later 

 for each degree of latitude towards the north. The same plants 

 flower at Zurich 6 days later than at Parma; at Tubingen, 13 days 

 later; at Jena, 17; at Berlin, 25; at Hamburgh, 33 ; at Greifswald, 

 36 ; and at Ohristiania, no less than 52 days later than at Parma. In 

 the Berlin district, an elevation of 1000 feet renders vegetation 10 to 

 14 days later : so also in regard to fruiting. The wheat harvest 

 begins 



At Naples . . In June. 



In Central Germany . . . July. 



In the South of England . . August. 



Kipe Cherries are to be had 



At Naples . . First days of May. 



At Paris ... . End of June. 



In Central Germany . . Do. 



In the South of England 22d July. 



