56G EFFECTS OF LIGHT, CLIMATE, AND SEASON. 



1 140. Light is an agent that has a powerful influence on plants, 

 as regards their vigour, irritability, secretions, and colour. Hence, in 

 those regions where the light is intense, the vegetation presents certain 

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

 of the woody matter deposited, of the fruit produced, and of the secre- 

 tions formed, are all influenced in some degree by the intensity of the 

 sun's rays. Little is known in regard to the effects of increased or 

 diminished atmospheric pressure on plants. The effects of the atmo- 

 sphere have been studied chiefly as regards dryness and moisture, and 

 the mixture of certain gases with it, especially in the vicinity of manu- 

 facturing towns (f 297). 



1141. 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 18. 



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 latitudes 

 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 Christiania, 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. 



Ripe 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. 



II. DISSEMINATION OF PLANTS. 

 1. AGENTS EMPLOYED IN THEIR DISSEMINATION. 



1142. Some plants are disseminated generally over the globe, while 



