8 



HISTORY OF THE VEGETABLE KINGDOM. 



Beaded vessels. The moni- 

 liform, or beaded tubes, fig. a, 

 are porous or punctuated, con- 

 tracted at different distances, 

 and crossed by partitions, 

 which are perforated with 

 holes like a sieve. These 

 vessels are chiefly found at 

 the junction of the root and 

 stem, and of the stem and 

 branches. 



Punctuated iiesseh. These, fig. b are continuous 

 tubes, on which are a number of opaque points ; 

 or, according to some, pores dis- g 



persed in transverse lines ; hence 

 Mirbel has called them porous 

 tubes. They are found in most 

 abundance in the stems of woody 

 plants, and particularly in wood 

 that is firm and compact, as the 

 oak ; but they do not, like the 

 simple tubes, seem destined 

 to convey any oily or resinous 

 juices. — See section of oak, Plate 



!,%•/• 



Slit vessels, or false spirals, fig. c. These are 

 tubes with a number of slits in a transveree di 

 rection ; they are very abundant 

 in the woody layers and fibres of 

 most species of vegetable produc- 

 tions, and serve, with the fore- 

 going, as capillary tubes, through 

 which the sap and juices of the 

 plant flow. These tubes are appar- 

 ently spiral on a slight inspection, 

 but upon more minute examination, 

 are found to derive this appearance 

 merely from their being cut trans- 

 versely by parallel tissues ; they cannot, conse- 

 quently, be uncoiledlike the true spiral tubes ; nor 

 can they be separated into distinct rings, because 

 the continuity of the membrane of which they 

 are formed, and consequently the extremity of 

 the fissure, which may always be discovered by 

 a little attention, prevent that separation. They 

 are somewhat similar to the porous tubes, for 

 the fissures, like the pores, are furnished vrith 

 a ring surrounding the top. But they are more 

 generally found in the soft parts of woody plants 

 than the porous tubes, and often also in the her- 

 baceous plants. In ferns they are found in great 

 abundance, and also in the soft parts of the 

 vine. 



The Spiral vessels, tig.d. These are fine, trans- 

 parent, and thread-like tubes, which are occa- 

 sionally interspersed among the other vessels of the 

 plant ; but distinguishable from them by being 

 twisted in a spiral form, either from right to left, 

 or the reverse, somewhat in the manner of a cork 

 screw. They are found in greatest plenty in herba- 

 ceous plants, and particularly in aquatic species ; 



but they are also to be met with in 

 woody plants, whether shrubs or trees. 

 If the stalk of a plant of the lily 

 tribe, or a tender shoot of elder, is 

 taken, and partly out across, and 

 then gently broken or twisted asun- 

 der, the spiral tubes may be seen even 

 by the naked eye uncoiled some- 

 what, but remaining still entire, even 

 after all the other parts have given 

 way ; and if the inferior portion 

 d of the stalk is not very large, it 



may be kept suspended for some considerable 

 time, merely by the strength of the tubes, 

 which, though now almost entirely uncoiled 

 by means of the weight they support, wUl, when 

 they finally break, suddenly wind up at each 

 extremity, and again resume their spfral form. 



Grew and Malpighi, who first discovered and 

 described them, fancied they resembled in ap- 

 pearance the trachea, or windpipe of animals ; 

 and hence described them by this name, under 

 which they are stiU very generally known. Du ' 

 Hamel endeavoured to convey an idea of their 

 form, by comparing it to that of a piece of rib- 

 band rolled round a small cylinder, and then 

 gently pulled off in the direction of its longi- 

 tudinal axis. The figure of the ribband becomes 

 thus loosely spiral. Tliis is a very good illus- 

 tration of the figure of the spiral tubes in their 

 uncoiled state ; but it does not represent them 

 very correctly as they exist in the plant. But 

 the best illustration of this kind is, perhaps, that 

 of Dr Thomson. Take a small cylinder of 

 wood, and wrap round it a piece of fine and 

 slender wire, so as, that the successive rings may 

 touch one another, and then pull out the cylin- 

 der. The wire, as it now stands, will represent 

 the spiral tubes as they exist in the plant ; and 

 if it is stretched by puUing out the two extremi- 

 ties, it will represent them in their uncoiled 

 state also. But although the spiral tubes are 

 to be met vsdth in almost aU plants, they are not 

 yet to be found in all the difierent organs of the 

 plant ; or at least, there are organs in which they 

 occur but rarely, or in very small numbers. 

 They do not seem to occur often in the root, or 

 at least they are not easUy detected in it. Grew 

 and Malpighi do indeed represent them as oc- 

 curring often in the root, the former referring 

 for examples to the roots of plants in general, 

 and the latter to those of the asparagus, poplar, 

 convolvulus, elm, and reed, all of which, says 

 Mr Keith, I have examined with great care, 

 without being able to discover any spiral tubes. 

 Sprengel states, however, that these spiral vessels 

 are always in the company of the sap vessels, 

 being chiefly found between the bark and pith 

 in common plants ; but they appear later than 

 the sap vessels, and are only discerned when the 

 young plant begins to shoot. They are, he adds, 



