SECONDARY PHLOEM 657 



time ; tliey lose their contents and become obliterated at the end 

 of each vegetative period at the latest. The cells of the conducting 

 parenchyma, on the other hand, may remain fully active for years. 



The mechanical portion of the secondary phloem typically consists 

 of bast-fibres and sclerenchymatous cells (sclereides). A considerable 

 number of plants form no secondary fibrous elements {e.g. Abies, Fagus, 

 Platanus, Viburnum); "stone-cells" (sclereides) are even more frequently 

 absent (CUPEESSINEAE, Taxus, Ulmvs, Morns, Lonicera, Samlucus, Bcr- 

 beris, Tilia, etc). It is, however, comparatively unusual for the secondary 

 phloem to be entirely devoid of mechanical cells (examples are furnished 

 by Lciurus, Nerinm, Cornus, Eibes, Buxus, etc.). 



In typical cases the secondary bast-fibres are arranged according 

 to one or other of the two following schemes. Either they are scattered 

 singly or in small groups throughout the secondary phloem, or they 

 form more extensive tangential layers or plates, alternating with similar 

 masses of leptome-tissue. The second of these two modes of arrange- 

 ment is exemplified by the Cupeessineae, and by Castanea vesca, Juglans 

 nigra, Vitis vinifera, Clematis Vitalba, Quercus Robur, etc.; where it 

 prevails, the secondary phloem is often intersected by large masses of 

 fibrous tissue. It would, however, be a mistake to suppose that this 

 mechanical tissue contributes appreciably to the inflexibility of the stem 

 as a whole. The strength of tree-trunks and large woody branches 

 depends entirely upon the mechanical properties of the woody cylinder ; 

 the fibrous portion of the secondary phloem accordingly serves almost 

 entirely to satisfy local mechanical requirements, or, in other words, to 

 protect the leptome-tissue with which it is associated. It does not follow, 

 however, that the presence of such fibrous layers is entirely without 

 effect upon the total mechanical strength of the stem ; their importance in 

 this respect will naturally be greatest, when the woody cylinder is feebly 

 developed to begin with. A twig of Lime, for example, may derive a 

 large proportion of its mechanical strength from the massive fibrous 

 strands in the secondary phloem during the first three years of its 

 existence ; but as continued cambial activity adds more and more to 

 the growth of the woody cylinder, the mechanical significance of the 

 bast becomes more and more strictly local. 



In addition to mechanical elements, the secondary phloem very 

 often contains crystal-sacs, and sometimes also secretory passages or 

 latex-tubes. These structures, when present, exhibit no special pecu- 

 liarities, and hence demand no further consideration. 



Finally, a few words must be devoted to the medullary rays of the 

 secondary phloem (phloem-rays). Among Dicotyledons the constituent 

 cells of these rays resemble the other conducting parenchyma elements 

 of the secondary phloem as regards their contents and the character of 



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