VASCULAR TISSUE. 45 



bast (Fig. 53) are so extremely thick-walled as almost to obliterate 

 the cavity. The disproportion in length is still greater in our 

 Leather-wood, which has a bark of extraordinary toughness, used 

 for thongs, while the wood is very brittle and tender. Its capillary 

 bast-cells measure from an eighth to a sixth of an inch in length, 

 with an average diameter of s ^ of an inch (so that, if the whole 

 length of a cell, magnified as in Fig. 54, 55, were given, the figure 

 would be from a foot to a foot and a'half in length) ; while those of 

 the wood itself are only the hundredth of an inch long. Among the 

 bast-cells are found the longest cells which occur in any tissue. Still 

 the individual cells are by no means absolutely so long as they are 

 supposed, and have sometimes been stated, to be. Few are of such 

 length as those of the Leather-wood, above mentioned. According 

 to Mohl (Bot. Zeit. 1855, p. 876) there are few plants in which 

 they exceed the twelfth of an inch ; but he has found them an inch 

 long in Flax and in our common Milkweed (Asclepias Cornuti), and 

 somewhat longer in the Nettle. 



56. Woody tissue runs lengthwise through the stem, root, or other 

 organ ; hence it is sometimes designated as Longitudinal Tissue, the 



Vertical or Longitudinal System of the stem, &c. It shares this 

 name, however, with some other forms of tissue Avhich accompany 

 it, particularly in the wood. The cells which compose it agree 

 in exhibiting markings of some kind on their walls, and in being 

 larger than those of woody tissue : they are all more or less tubular, 

 or conspire to form tubes of considerable length, and hence they have 

 all been combined, in a general way, under the name of 



57. Vascular Tissue or Vessels. Not to be misled by the name, it 

 should be remembered that these so-called vessels are mere modifica- 

 tions of cellular tissue, and are wholly unlike the veins and arteries 

 of animals. It is much better to call them ducts, a name appropriate 

 to their nature and office, and leading to no false inferences. Their 

 true nature is most readily shown in the largest and most conspicu- 

 ous kind, one which often exhibits unequivocal indications of its 

 cellular origin, viz. 



58. Dotted Ducts, called also Pitted or Vasiform Tissue, Boihren- 

 chyma, &c. (Fig. 56, 57). They have likewise been termed Porous 

 Cells or Porous Vessels ; but the numerous dots that characterize 



them are places which have not been thickened in the manner 

 already explained (41, 44), and not perforations, except in old cells, 

 where the primary membrane may be obliterated. Sometimes they 



