PHYSIOLOGY. 



259 



Fig. 228. 



about the nineteenth of an inch square, Lewenhoeck, with his 

 powerful microscope, saw 20,000 of these vessels. 



CONDUCTING VESSELS. It was formerly supposed that ali 

 the vessels which Cotyledonous vegetables contain were 

 modifications of the spiral form ; but more recent discoveries 

 have shown that there exists another kind, which are com- 

 posed of rings, and which cannot be a transformation of the 

 spiral form. These are called annular, or ring-shaped vessels. 



There are, therefore, two primary forms of organic vege- 

 table tissue, the spiral arid the annular, and of which we shall 

 see that all the other forms are modifications, or varieties. 



Simple Spiral. If the petiole of a Dog- wood, or Elder- 

 leaf, be carefully broken, and the parts drawn asunder, the 

 spiral vessels may be seen with the naked eye, having the 

 appearance of threads ; but which on closer examination, will 

 be found to consist of one or more fine 

 silvery fibres turned from right to left, in 

 the form of screws, so as to make hollow 

 cylinders. Sometimes the spirals are 

 formed of a single thread, as represented 

 at a, Fig. 228, while at others, or in other 

 plants, they consist of several parallel 

 fibres, forming a ribbon, the edges of which 

 are wound in contact with each other, as 

 shown at b, in Fig. 228. 



These fibres are tenacious and elastic, 

 for when stretched and unrolled, they con- 

 tract and roll themselves up again, when 

 the force is withdrawn. 



All the higher orders of vegetables contain these vessels, 

 and in which they may be detected in the earliest stages of 

 growth. They run through the whole length of the plant, 

 from the extremities of the roots to the leaves, following all 

 the various curvatures of the branches to the smallest parts. 



These vessels are generally disposed in fasciculi, or bun- 

 dles, the smaller ones of the group being always found next 

 to the pith. The cells, which as we have seen, are found 

 with the spiral vessels, when in the vicinity of these bundles, 

 are very small, but there is no evident communication be- 

 tween the two kinds of vessels. 



In succulent plants, the spiral vessels are found in the 

 cellular pulp, or parenchyma. In woody plants they always 



