190 



PART II. VEGETABLE HISTOLOGY. 



spicuously different in color from the rest, owing to infiltrated 

 coloring matter. This portion of the wood, which has ceased 

 to take any important part in the vital processes of the plant, 

 is called duramen, or heart-wood, while the exterior zone of 

 thinner-walled, uncolored and active cells is called alburnum, or 

 sap-wood. 



Rings of growth are observed in the wood of trees that inhabit 

 climates where there is a decided change of seasons, so as to 

 produce periodical cessation of growth. 



These rings are formed by the juxtaposition of the small cells 

 formed in the latter part of the season of growth, to the larger 

 ones formed at the beginning 

 of the succeeding season. This 

 will be understood by reference 

 to Fig. 452. In climates where 

 there is during the year one 

 period of growth and one of rest, 

 there will usually be formed one 

 ring each year, and the number 

 of rings in the wood, therefore, 

 becomes an index, approxi- 

 mately, at least, of the age of 

 the stems. Sometimes, how- 

 ever, rings are due to other 

 causes. Periods of drouth in 

 midsummer, accidental losses 

 of large branches possessing a great amount of leaf-surface, or 

 other causes seriously interfering with regular growth, may result 

 in the formation of two or more rings during a season. They 

 must not be relied on, therefore, as indicating with absolute 

 accuracy the age of trees. 



The bark, also, in its inner layer, frequently exhibits rings of 

 growth due to similar causes. The bark of Dicotyledons and 

 Gymnosperms, when all the parts are present, consists of four 

 layers, the epidermis, the epiphlceum or corky layer, the meso- 

 phloeum or green layer, and the endophloeum or liber. 



The epidermis has already been described. In perennial 

 stems it seldom persists for more than two or three years. 

 When it first reaches maturity it forms, except where the stomata 



Fig. 452.— Small portion of transverse 

 section of wood of White Pine, showing part 

 of a ring of growth, a. The small cells at a 

 were produced near the close of the season's 

 growth, and the large cells immediately to 

 the left were produced in the spring of the 

 succeeding season. 



