CHAPTER III. ORGANS OF PLANTS. I93 



hence, in the large trunks of many of our forest trees all tissues 

 exterior to the liber have disappeared, and, in the strict sense of 

 the term, nothing of the bark but the inner layer or endophloeum 

 is left. 



The liber is always present and always constitutes the most 

 important portion of the bark. Although, as we have seen, its 

 outer layers may peel off while being renewed from within, it 

 always consists, when first formed, of the phloem of the fibro- 

 vascular bundles, together with that portion of the medullary rays 

 which separate these parts of the bundles laterally. It consists, 

 therefore, largely of sieve and parenchyma tissues, with hard bast 

 often intermingled. It not infrequently also contains other 

 tissues, such as milk tissues, secretion cells, etc. The soft tissues 

 of this layer are particularly rich in albuminoid and other 

 nutritious matters, and in medicinal barks it constitutes the area 

 in which the active principles are usually found in greatest 

 abundance. 



Since in perennial stems this layer increases in thickness 

 year by year by growth in the cambium, it often, though not 

 always, presents the phenomenon of rings of growth similar to 

 those seen in the woody part of the stem. 



It is not always the case that the stems of Dicotyledons con- 

 form fully to the type which has been described. Cucurbitaceous 

 plants, like the Pumpkin and Melon, have, as we have seen, 

 bi-collateral bundles ; moreover, these bundles are arranged in 

 two concentric circles instead of one. Other curious deviations 

 from the typical form occur in the stems of many tropical 

 climbers. 



The stems of Gymnosperms show, for the most part, the same 

 arrangement of the fibro-vascular bundles as those of Dicotyle- 

 dons. They differ from the latter mainly in the fact that the 

 tissues, particularly those of the xylem, are less complex. In 

 most cases the ducts and proper wood-cells which are so abun- 

 dant in the xylem of Dicotyledons are few in number or altogether 

 wanting, these tissues being replaced, as we have seen, by disci- 

 gerous tracheids. 



Histology of the Leaf. The leaf consists of: (i) the 

 fibro-vascular system, or framework, which was partly described 

 in Part I, under the head of •* venation ; " (2) the parenchyma. 



