CHAPTER VIII 

 THE FIBROVASCULAR TISSUES PHLOEM 



The ligneous or woody tissues which have been discussed under 

 various headings in previous chapters are of great importance on 

 account of their conservatism. They manifest a high degree of 

 differentiation and are also important by reason of the fact that 

 their relative imperishability has resulted in their being abundantly 

 preserved as fossils from the most ancient times to the present. 

 Woods consequently furnish on the whole the most important 

 historical document in favor of the hypothesis of evolution, and 

 those who are interested in the doctrine of descent as applied to 

 the case of plants cannot afford to neglect the investigation of 

 the ligneous tissues of the vascular plants. The situation for the 

 plants provided with tracheary tissues is, in fact, very different 

 indeed from that presented by the lower vegetable organisms, where 

 evolutionary data are necessarily largely speculative and experi- 

 mental. Invariably associated with the woody tissues are those of 

 the phloem. The primal function of the woody tissues was appar- 

 ently to conduct water into the superior or outlying parts of the 

 plant. To this function have been superadded in the long course 

 of geologic time, as pointed out in preceding chapters, the functions 

 of strength and storage, with corresponding and appropriate 

 modifications of structure. Just as the wood in the first instance 

 served the purpose of the movement of water from the soil, so the 

 primal condition of the tissues of the phloem was that of conducting 

 the elaborated foodstuffs manufactured in the leaves to other 

 parts of the plant where they are either utilized in the processes 

 of growth and respiration or stored up as reserves. 



The primitive type of phloem, as that of xylem, appears to have 

 existed in the case of the lepidodendrids. Its structure has not 

 been satisfactorily preserved in these primitive vascular plants, and 

 there is some difference of opinion as to interpretation. Some 

 investigators have expressed the opinion that no true phloem is 



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