ERECT FOLIAGE STEMS. 719 



sunlight, and frequently curve up towards the light, as may be well seen in ash 

 and chestnut-trees, and also in the spruce firs represented on p. 415. 



The lower portion of the stem which has lost its boughs increases in circum- 

 ference as the burden it has to bear becomes greater, and its thickness and strength 

 in every species bears a definite relation to the weight of the crown. The increase 

 of circumference is brought about by the addition each year of new masses of wood 

 to those already present. In very young stems the wood appears in the form of 

 strands, symmetrically arranged round the central pith, closely adjoining one 

 another and forming a cylinder which is only interrupted by the medullary rays. 

 The annual increments of wood, deposited on the periphery of this primary ring, 

 also have the form of rings in cross section; each is known as an annual ring. 

 The age of a felled tree can be reckoned from the number of these annual rings, 

 and obviously the girth of the stem increases with their increasing number. The 

 enlargement of the circumference is, however, not without its effect on the external 

 appearance of the stem. While still quite young, the stem possesses a covering 

 skin (epidermisj which closely surrounds the green tissue of the cortex. This 

 epidermis, however, only keeps pace with the development of the interior of the 

 stem as long as this particular part continues to grow in length. When it stops, 

 and increase in thickness commences, the first skin perishes, and is replaced by a 

 second, the so-called 'periderm. This usually begins to develop even at the end 

 of the first period of vegetation. The most important constituent of periderm is 

 cork, a tissue of cells impervious to water and almost impervious to air, and there- 

 fore excellently fitted as a covering for the inner sap-conducting portions of the 

 stem. Whatever lies outside this cork, or is secreted through it from the inner 

 sap-containing portions, dries up and dies. If the periderm is developed immediately 

 beneath the epidermis, this alone perishes; but if the periderm arises in the deeper 

 layers of the cortex, a considerable thickness of cortex also dies and remains outside 

 the cork as a dead dry crust. This inner periderm with the dead adhering parts 

 of the cortex is called the hark. 



The development of the periderm keeps pace with the development of the stem. 

 As soon as the wood of the stem becomes thicker, by the intercalation of a new 

 annual ring, the mantle of periderm stretches, and consequently the whole envelope 

 of bark. In many trees this bark remains year after year on the periphery of the 

 stem; it becomes fissured by the continuous increase in thickness, but new bark is 

 as continuously produced from within closing up the fissures. In other instances 

 a part of the bark falls ofi" on to the ground in consequence of the thickening of the 

 stem, and is again replaced by new bark from within. 



Since every kind of tree has its own special bark, the texture and colour of 

 this structure contributes not a little to the appearance of the whole tree; it 

 forms one of the characteristic features which must not be overlooked when 

 describing the habit of the tree. The following are the most important forms of 

 bark. First the scale bark, which is detached annually in the form of shields 

 and plates, to be seen especially well in the stems of planes, almond willows. 



