32 ORGANOGRAPHY. 



long tubular cells placed end to end, with very thin walls, are 

 by no means so strong as those made from woody tissue. 



' The diiferent kinds of woody tissue arc commonly associated 

 in the jDlant with other organs also of anelongated tubular cha- 

 racter, but larger than the prosenchymatous cells of which they 

 are composed. These constitute the 



3. Vessels or Vascular Tissue. — These names were origi- 

 nally given from an erroneous idea of their resemblance to the 

 vessels of animals, with which however, tliey have no analogy. 

 The name of duct is also frequently applied to them by au- 

 thors. By some writers again, a distinction is drawn between 

 a vessel and a duct, the former term being used to represent a 

 long tubular cell with tapering ends, having spiral deposits 

 in its interior; while the latter is used to indicate an elongated 

 tubular body, which is formed of a row of ordinary cylindrical 

 cells applied end to end, the partitions- between the cavities of 

 which have become absorbed so as to form a continuous canal. 

 There are several varieties of vessels or ducts, the nature of 

 which depends upon the modifications which their walls undergo 

 by secondary deposits in their interior. Thus we have pitted, 

 spiral, annular, reticulated, and scalariform ducts, or vessels as 

 we shall in future call them, as more in accordance with general 

 custom. So long, however, as their nature is remembered, it 

 is of little consequence which term is used. 



a. Pitted or Dotted Vessels. — These constitute by their com- 

 bination the Pitted or Porous Tissue of some authors, or the 

 Vasiform Tissue, Bothrenchyma, or Taphrenchyma of others ; 

 the two latter names being derived from Greek words signify- 

 ing pits. They either consist of elongated pitted cells with 

 pointed ends {fig. 9); or, as is generally the case, of a row of 

 cylindrical pitted cells placed end to end, the intervening parti- 

 tions of which have become more or less absorbed, so that their 

 cavities form a continuous canal {figs. .59 and 60). The origin of 

 pitted vessels from a row of cells of a similar pitted nature, is 

 clearly shown in many instances by the contractions which their 

 sides exhibit at various intervals, by which they acquire a beaded 

 or jointed appearance {figs, i^g and 60) ; for these joints evi- 

 dently correspond to the points whei-e the component colls 

 come in contact, and in some cases even we find the intervening 

 mcml)raue not comi)lctcly absorbed l)ctwccn the cavities, but re- 

 maining in the form of a network or sieve-like partition {fig. 61). 

 In the stems of herbaceous plants we may sometimes suc- 

 ceed in separating these vessels into their com])onent parts, 

 and thus clearly show tlieir origin from cells. Pitted vessels 

 generally terminate obliijuely {fig. 61), and, when they combine 

 with neighbouring vessels, the ul)li(iuc extremities of the latter 

 are so i)laced as accurately to correspond with the former, in 

 some cases, however, where the i)itted vessels arc pointed at the 



