470 



Mr. Hinds on Climate in connexion 



blue, the latter receiving a slight portion of yellow. The va- 

 ried colours of flowers, fruits, and of vegetable substances 

 generally are dependent on a peculiar and inscrutable pro- 

 perty of their tissues, enabling them to reflect certain rays or 

 portions of rays, the depth and richness depending on the 

 quantity of their exposure to heat and light. 



Though colour is one of the most prominent effects of light, 

 it materially influences some of the vital functions of plants. 

 Under it alone takes place the decomposition of carbonic acid, 

 whereby solid carbon is bestowed on the plant, and oxygen 

 yielded to the atmosphere. When deprived of light the tis- 

 sues are extremely lax, the whole plant assumes an unhealthy 

 longitudinal development, apparently searching for what is so 

 highly necessary to its economy ; the proper secretions are 

 not at all, or most sparingly, eliminated, losing the custom- 

 ary sapid, acrid, aromatic or other properties. Its effects on 

 many of the leaves and flowers of warm climates are remark- 

 able, both only expanding under its influence, and closing or 

 folding up as this declines or is removed. Pinnate leaves are 

 more particularly affected in this manner, and they greatly 

 prevail in low latitudes : many flowers are liable to this, Com- 

 posite present numerous instances ; the Oxalidea and Mesem- 

 bryaniherna of Southern Africa require the full glare of the 

 sun before they will expand their flowers, often opening only 

 for a short period, and when a cloudy day occurs they will not 

 unfold at all. The scarlet pimpernel (Anagallis arvensis) of 

 our fields has received the appellation of the shepherd's wea- 

 ther-glass, from the sensitiveness it betrays on the approach 

 of a cloud threatening rain, and the term appears to have been 

 applied with much correctness. Those flowers -with a con- 

 torted aestivation are eminently affected by its presence or 

 absence. 



Like temperature, the intensity of light diminishes from the 

 equator to the poles, but not so rapidly, rather corresponding 

 with the radiation of the sun's rays ; thus in high latitudes, 

 the light is proportionately greater for the temperature than 

 in low, and a similar circumstance happens in alpine situations, 

 as was observed long since by Saussure. The protracted pe- 

 riod in northern climates at one time of the presence of light, 

 and at another of its absence, might be supposed to affect pro- 

 portionately the vegetation ; a flora may be expected running 

 its course within a short period, displaying considerable vigour 

 of growth, and after having perfected the seeds, leaving future 

 existence entirely with them. In a somewhat lower latitude 

 careful preparation is made to meet that portion of the year 

 when the temperature is rigorous and light greatly removed ; 



