INTRODUCTORY. 45 



" Next to the cambium, and united to the wood by the rays 

 from the pith, is the bark, consisting of three Layers." 



"The inner, or fibrous layer, is formed by l)ast cells and 

 firm cellular tissue. Surrounding the inner bark is a layer of 

 cellular tissue in which the rays from the pith terminate, and 

 which is named the green layer, l)ecause it often exhibits this 

 color in young shoots, and then performs the same function 

 with the green tissues of the leaf. Outside of all this is the 

 corky layer, consisting of dry, dead, cellular tissue, and devel- 

 oped annually from the green la3'er. This is not usually of 

 much thickness, or consequence, but sometimes, as in the cork 

 oak of Sixain, it becomes an important article of commerce." 



'■ The growth of our trees goes on in the cambium layer, 

 from which is produced annually a layer of wood and a layer 

 of bark, each formed of longitudinal fibro-vascular tissue and 

 horizontal cellular tissue." 



" As the trunk expands, the outer bark cracks and falls off, 

 as in the shag-bark hickory, or distends and envelops it with 

 a somewhat smooth covering, as in the beech and birch. In 

 these latter cases the annual cortical layers are quite thin, and 

 the outer layer very gradually wastes away, under the influ- 

 ence of winds and storms. In the cork oak the outer layer is 

 specially thickened, and if removed every eighth year, may l)e 

 obtained in stout, elastic sheets, which would crack and tall to 

 the ground in the process of time if not harvested. The 

 structure of the root is not unlike that of the stem, except 

 that the pith is usually wanting, as well as the green layer of 

 the bark, which could not be formed nor be of any use in the 

 dark earth where the root makes its home." 



Fig. 8 represents a section, both vertical and horizontal, of 

 a branch of sugar maple, two years old, as it appears in 

 December. The portion included in the lines marked A is of 

 the first year's growth ; those marked B indicate the wood of 

 the second year ; while those marked C include the three 

 layers of the bark. D represents the pith of loose cellular 

 tissue ; E the pith rays of silver grain of hard cellular tis- 

 sue connecting the pith with the green or middle layer 

 of bark, which also consists wholly of cellular tissue. F 

 marks the outer or corky layer of the bark, which is 

 composed of dry, dead cells, which are formed of con- 

 secutive layers from the outer portion of the living green 

 layer; G is the green layer of cellular tissue. H shows the 

 liber or inner bark, made up of cellular tissue penetrated by 



