156 THE BOOK OF NATURE STUDY 



calculated according to tables which have been worked out, and 

 are usually supplied with the instrument. 



Wooded districts tend to make a climate more equable. Their 

 temperature is higher during the night and lower during the 

 day, the difference being most marked in summer and autumn. 

 Trees vary very much in their effect on temperature ; the beech 

 has double the effect of the spruce in July ; on the other hand, 

 the spruce has a far greater effect in winter than the beech. One 

 result of this more equable climate in the neighbourhood of 

 woodlands is, that vegetation does not suffer so much from ex- 

 tremes of weather, from frost or drought. 



EFFECT OF LIGHT ON UNDERGROWTH 



In studying the vegetation of woods, the first thing to deter- 

 mine is the nature of the wood, whether it is a pure or mixed 

 wood, whether there is coppice or not. The closeness of the 

 trees to each other ; the height of their boles, that is to say, the 

 height from the ground at which the lowest branches begin ; 

 the thickness of the foliage, should all be carefully noted. Then 

 the influence of the tree on the shrubby and herbaceous under- 

 growth may next be observed. The shrubby undergrowth often 

 consists of climbing plants that can straggle out to the light ; 

 Blackberry, Honeysuckle, Wild Roses, are very constant in woods. 

 The thickness of the shrubby undergrowth in its turn affects the 

 profuseness of the herbaceous vegetation, for the factor which per- 

 haps more than any other determines the richness and variety of 

 the undergrowth is light. The intensity of light is less important 

 than its duration. Warming states that in Finland barley ripens 

 in eighty-nine days, whilst a little farther south it takes a hun- 

 dred days, although the rays of light there are stronger ; but in 

 Finland the days are longer, and the barley is therefore exposed 

 to longer light per day, and this greater duration of light more 

 than counterbalances the difference in the strength of the light. 

 The intensity of light in a wood varies with different parts of 

 the wood, with the denseness of the foliage of the dominant tree, 

 with the thickness of the shrubby undergrowth. An easy way 

 of measuring this is with an ordinary exposure meter, such as 



