CHAPTER XXVII 

 THE WOODY DICOTYLEDONS 



As a matter of convenience the anatomical organization of the 

 woody dicotyledons will be considered in the present chapter. 

 It must not be supposed, however, that such a mode of treatment 

 implies that the woody texture of the stem has any value from the 

 phylogenetic or taxonornic standpoint. Perennial dicotyledons 

 are distinguished by the possession of a thick woody cylinder 

 resulting from the activity of a cambial layer situated between 

 phloem and xylem; this adds largely to the wood and less copiously 

 to the inner bark during the periods of growth. The woods of the 

 dicotyledons offer a great range of structural organization, and 

 their identification on the basis of anatomical features is corre- 

 spondingly difficult. 



In all except a few instances dicotyledonous woods are pro- 

 vided with vessels. These belong to two main types, namely, 

 those with scalariform and those with porous perforations. It 

 has been made clear in an earlier chapter dealing with the structure 

 and organization of the vessel that the type with scalariform per- 

 foration of the inclined terminal walls results from the fusion of 

 rows of opposite pits which gives rise to elongated horizontal 

 pores from which the membranes quickly disappear. This process 

 repeated in successive rows of pits results in the appearance of 

 lattice-like or scalariform perforations in the ends of the vessels, 

 and these permit a ready passage of water. On general evolution- 

 ary principles the vessel with scalariform perforations is to be 

 regarded as more primitive than the porous type presently to 

 be discussed. It is not surprising for that reason that it is a 

 characteristic feature of groups which are considered on good 

 grounds to be low in the dicotyledonous scale. The vessel with 

 the porous type of perforation is clearly derived, as has been dem- 

 onstrated in an earlier chapter, from the scalariform condition, 

 in the first instance at any rate, by the loss of the bars of 



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