36 DISEASES OF TREES 



it is well known that the stems of beeches grown on the best 

 soils, especially such as are calcareous, bear but few lichens, 

 whereas on the poorer soils, especially such as are sandy, lichen- 

 covered stems are very abundant. When a beech grows very 

 rapidly in thickness, the formation of periderm* must also be 

 rapid, and, since the dead cork-cells on the outer side of the cor- 

 tex are soon exfoliated and pushed off, luxuriant development 

 of lichens is impossible. Where growth in thickness progresses 

 very slowly, the dead cork-cells adhere to the cortex for a much 

 longer period, so that lichens are enabled to grow longer and 

 develop more vigorously. Under such circumstances, too, mois- 

 ture is longer retained, and this also favours the growth of 

 lichens. The same remarks apply to trees, such as the spruce, 

 whose outer layers of periderm are cast off as scales, or whose 

 moribund layers of cortex are thrown off in later life as plates 

 of bark. The slower the growth of a tree, the more slowly do the 

 outer cortical layers die, and so much the more suitable are the 

 conditions for the growth of lichens. Consequently, although the 

 presence of lichensis primarily the sign of a permanently humid 

 atmosphere, or of a slow rate of growth, it cannot be denied 

 that they do some small amount of damage to trees. During 

 summer the tree takes in oxygen by means of numerous lenticels, 

 and this process goes on even in the older parts of the stem. 

 The presence of oxygen in the interior of the tree is absolutely 

 necessary for maintaining the processes of metabolic, or chemical 

 and vital changes. Now, if the passage of oxygen to the lenti- 

 cels f of the cortex is impeded by a luxuriant growth of lichens 

 or mosses, we may assume that the tree suffers more or less in 

 health. This may furnish us with a reason for the death of so 

 many branches of spruces and larches whose crowns are over- 

 grown with lichens. 



* [Periderm is the corky covering which replaces the delicate epidermal 

 layer as the stem grows older. Cortex is the green, living cellular tissue 

 covered by the periderm, &c. For the connection between "periderm " and 

 " bark," see later in Part II. On a two-year-old twig of a tree, about June or 

 July, we usually find epidermis on this year's growth, and periderm on the 

 browner, older part. ED.] 



f [Lenticels are the perforated or pervious corky warts noticed on the 

 periderm of twigs e.g., they are very evident on twigs of chestnut, elder, 

 &c. ED.] 



