48 



THE ELEMENTARY STRUCTURE OF PLANTS. 



56. The woody tissue runs lengthwise through the stem, root, or 

 other organ (except in reticulated leaves, and there its ramifica- 

 tions all spread in one plane) ; for this reason, it is sometimes des- 

 ignated as Longitudinal Tissue, the Vertical or Longitudinal Sys- 

 tem of the stem, &c. It shares this name, however, with some 

 other forms of tissue which accompany it, particularly in the wood. 

 These all agree in exhibiting markings of some kind on their 

 walls, and in being larger than 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, This is an unfortunate name, how- 

 ever, and apt to mislead, like most of those in botany that are 

 based on loose analogies with the animal kingdom. To avoid or 

 correct the erroneous impressions that are so prevalent, it should 

 be remembered that these so-called vessels are comparatively un- 

 essential modifications 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 conspicuous form, which often exhibits unequivo- 

 cal indications of its cellular origin, namely, 



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

 chyma, &c. (Fig. 38, 39). They have likewise been termed Po- 

 rous Cells or Porous Vessels ; but the round or 

 oblong dots that characterize them are thin 

 places where the wall has not been thickened 

 by an internal incrusting deposit, as has al- 

 ready been explained (44), and not perfora- 

 tions, except in old cells where the primary 

 membrane is obliterated at these points. Some- 

 times they are continuous tubes of considerable 

 length (Fig. 40) ; but commonly, the circular 

 lines which they exhibit at short intervals (as in 

 Fig. 39), and the imperfect transverse partition 



which is often found at these points, plainly indicate their compo- 

 sition ; showing that they are made up of a row of cells, with the 

 intervening partitions more or less obliterated. In Fig. 21, some 



FIG. 39. Portion of a dotted duct from the Vine, evidently made up of a series of short cells. 

 FIG. 40. Part of a smaller dotted duct, showing no appearance of such composition. 



