56 COMPOUND ORGANS OF PLANTS. 



gives to young shoots their green hues, as it shines through the 

 transparent membrane, the cuticle. In the shoots of young 

 trees, in the spring, it may be readily perceived. The bark of 

 a young stem the first year always assumes this color, from 

 the production of chlorophyl in its superficial cells, owing to 

 their direct exposure to the solar light. 



The mesophloeum or green layer of bark, does not grow at 

 all after the first or second year. It is excluded from the light 

 by the gradually exterior deposition of layers of epiphlceum, 

 and finally perishes never to be renewed again. 



4. The endophlmum or inner bark, called also the liber, ff. 

 This constitutes the fibrous portion of the bark, the corky and 

 cellular envelopes being composed exclusively of cellular tissue. 

 It is in the fibrous portion of the bark that the sap vessels are 

 contained, which convey the sap from the roots to the highest 

 extremities of the plant ; hence the endophlceum continues to 

 grow throughout the life of the plant, being formed in conjunc- 

 tion with the alburnum or sapwood directly from the cambium 

 layer. 



The endophloeum or inner bark possesses considerable 

 strength and many useful properties. The inner layers of 

 Tilia Europoea, or the lime tree, when separated by maceration 

 in water, form the common bass or matting used by gardeners, 

 and the woody fibre which is used for the manufacture of 

 cordage in all exogenous plants, as in hemp, flax, &c., belongs 

 to the endophloeum or inner bark, and not to the wood. 



The cambium layer, e e. This layer has been already 

 described, pp. (50-1). In herbaceous plants it is not able to 

 organize itself, because the stem dies down to the ground the 

 first year. In every other respect the herbaceous stem offers 

 the same structure as the ligneous, being composed equally of 



