OR, PLAIN 



and having their fibres twisted 

 like a lady's ringlet. They are 

 not of great length, but are united 

 together by their extremities, 11. 

 They are met with in all parts of 

 plants except the roots, but more 

 particularly surrounding the pith, 

 and in all parts emanating from it. 



They may be readily obtained by cutting 

 through the cuticle of the footstalk of the straw- 

 berry leaf, and then gently separating the di- 

 vided portions, when they appear as a very fine 

 thread, arranged in loose spires. They abound 

 in the veins of leaves, and even in the minutest 

 parts of flowers. Perhaps, of all positions in 

 which the spiral vessels may be the best in- 

 spected, that of the veins over the brown coat 

 of the common hazel nut, after the shell has 

 been removed, is the most accessible. The 

 brown membrane should be soaked in water for 

 a short time, and then the veins carefully torn 

 open with needles, and placed under the micro- 

 scope. They are seen to great advantage also 

 in certain succulent stems, as those of the pota- 

 toe, by cutting the stem across obliquely with a 

 blunt knife — the section being placed under the 

 microscope. The use of spiral vessels has been 

 the subject of much investigation, and it appears 

 probable that at some period they convey air 

 highly charged with oxygen, and thus promote 

 a system of internal respiration, like that effected 

 by the air tubes in insects.* 



The manner in which the 

 vessels are distributed in bundles 

 through the cellular tissue is 

 shown by microscopic inspection 

 of a cutting of the brake-fern, 12, 

 12 



the vessels, 13, being surrounded 

 by the cellular tissue, 14. 



From the extreme minuteness of the vessels, it 

 is scarcely possible to compute their number 

 with accuracy. In a piece of vegetable texture 



* Orr'a Circle of the Sciences. 



TEACHING. 195 



of a square inch, it has been estimated that 

 there were no fewer than 7,290,000 vessels. In 

 an oak tree of no more than one foot in circum- 

 ference, it is estimated that the vessels amount 

 to 200,000,000. But f he number varies in plants 

 of different structure. They are most numerous 

 in hard woods, and least so in spongy and juicy 

 stalks. In some pi ants they are large enough 

 to he discerned by the naked eye. 



These vessels convey the sap 

 by which the plant is nourished, 

 and they have received different 

 names, according to peculiarities 

 of structure which they exhibit. 

 One set of vessels receive and 

 carry sap to the leaves, and 

 another set bring it back from 

 the leaves to the baric. Thus 

 there is a kind of circulation in 

 plants. 



The moisture f>f the soil is 

 absorbed by the ro#ts, 14 a; it then 



549. 



enters the vessels occupying the 

 central parts of the woody stem, 

 15; having traversed the trunk 

 it enters the branches, 16, and at 

 last reaches the leaves, 17; here 

 it combines with the gases of the 

 atmosphere through the pores of 

 the leaves, which lie principally 

 h ? 



