THE SAXIFRAGES 77 



solution is secreted by the leaf, and this exudes at the 

 pores. As the water evaporates, the chalk crystallises 

 out, and blocks the mouth of the pore. In warm 

 weather and in direct sunlight, the opening of the 

 pore is almost completely closed in this manner. At 

 night, however, when the temperature is lower, more 

 water is secreted, which dissolves some of the calcium 

 carbonate, and thus a freer passage for the gaseous 

 exchange is afforded. In this way the chalk-glands 

 control the rate at which the leaves lose water to 

 the atmosphere. 



It is a curious fact that Saxifrages with chalk-glands 

 may often be found growing in abundance on rocks 

 composed of granite or on schists, which contain very 

 little or no lime. Yet by means of their roots these 

 plants can obtain from the soil suflBcient lime or 

 chalk to render the incrustation-mechanism of the 

 leaves quite efficient. The roots of plants have a 

 peculiar property, known to botanists as "selective 

 capacity," which enables them to gather in or absorb a 

 sufficient quantity of a substance in the soil, even 

 when it exists only in extremely minute quantity. 

 Thus plants can appropriate, if they need it, a 

 considerable quantity of one particular substance, to 

 the exclusion of others. For instance, sea-weeds 

 absorb from sea- water sufficient phosphorus — one of 

 the essential elements to the life of all plants — though 

 the amoimt of phosphorus in sea-water is so exces- 

 sively small that it is quite impossible to estimate it. 

 Closely allied to the Evergreen Saxifrage is the Thick- 



