XVIII 



ON THE CHALK 



While some flowers are so universally distributed that they may 

 be described as existing almost everywhere, others are restricted 

 to certain kinds of localities, outside which they seldom occur. 

 This restriction is sometimes merely one of light and shade, the 

 same species growing almost equally luxuriantly in open spaces, or, 

 in shady places, regardless of other conditions. Some plants, 

 however, are particularly partial to certain conditions of soil, 

 situation, or cUmate, and are consequently more strictly confined 

 to hmited districts. 



We have aheady referred to several species which are essentially 

 flowers of the woods, but even these are not distributed evenly in 

 wooded districts ; for while some seem to be more universally 

 scattered throughout our wooded parts, others show a decided parti- 

 ality to particular soils, being found exclusively, or almost so, 

 either in sandy woods, clayey woods, or woods in limestone dis- 

 tricts, &c. In fact, the nature of the soil is such an important 

 factor in determining plant distribution that we naturally associate 

 many species with the particular rock strata on which we almost 

 invariably find them. 



So intimately is the distribution of plants connected with that 

 of the geological strata that when, in the course of a day's ramble, 

 we find a more or less sudden change in the nature of the flora, we 

 may be almost sure that there is a corresponding change in the 

 nature of the rocks or soil over which we have strayed; and the 

 young botanist wiU find much to interest him in the study of this 

 relation between vegetable life and geological structure. Of course 

 we do not mean that the botanist must necessarily be also a geologist, 

 but that he should, at least, be always ready to observe the nature of 



