58 FORMS OF BOTHRENCHYM. [BOOK i. 



C. When, with a more intimate dependence of the tubes 

 on the cells, the walls abutting on other vessels remain 

 thickly studded with pits ; only those in contact with prosen- 

 chymatous cells present very remote dots, or (at least for 

 considerable distances) none at all. The parts bordering on 

 the medullary rays have simple pits. Such tissue occurs in 

 Sambucus nigra, Betula alba, Aralia spinosa, Corylus 

 Avellana, Populus alba, Alnus incana, Platan us occidentalis, 

 Pyrus Malus, Gymnocladus canadensis. 



D. When contiguous cells, which have more commonly 

 the form of parenchym than prosenchym, exhibit pits 

 surrounded by a border on those portions of the walls only 

 which abut on other tubes ; those portions, on the contrary, 

 abutting on cells have numerous, large, perfectly borderless 

 pits, resembling those of the parenchym e. g. Cassytha glabella, 

 C. filiformis, Bombax pentandrum, Hernandia ovigera. 



E. There is a mere modification of this structure, but a 

 very peculiar one, when the walls which press on another 

 tube are fashioned like scalariform vessels, in consequence 

 of the pits being drawn out into fissures which extend the 

 whole breadth of the tube, while those walls which are con- 

 tiguous to cells only produce large borderless pits. This 

 form is beautifully developed in Chilianthus arboreus and 

 Cynanchum obtusifolium. In a less degree the same phse- 

 nomena are exhibited by Vitis vinifera in the walls contiguous 

 to vessels. 



Most bothrenchym may be referred to one of the heads 

 just enumerated. There is, however, in addition, a series of 

 forms in which the intervals between the rows of pits are not 

 level, but marked on the inside wall with a spiral line. 



These tubes are to ordinary bothrenchym what the dotted 

 tubes of Taxus are to other Conifers. In this kind of tissue 

 not only do we find variations in the distribution of the pits 

 similar to those just enumerated, but other differences occur, 

 depending upon the presence of spiral threads in a part or the 

 whole of the tubes. In some plants we may, for instance, 

 distinguish, though not very accurately, greater and smaller 

 tubes which are not always alike forming bundles, more 

 especially on the inner part of annular rings ; and near such 



