18 FIBRO-VASCULAR TISSUE. 



two surfaces. Other young branches will answer the same pur- 

 pose, some equally \vell with the one named, as the Asparagus,, 

 Strawberry, Currant, Dogwood, (fee. In the above cases the ves- 

 sels are not observed in their natural state, since they are seen 

 uncoiled. In the stem, the fiber that we see uncoiled, when 

 pulled apart, forms a complete tube by its edges coming in con- 

 tact in coiling. But if either the Pokeweed or Asparagus be 

 boiled, they may then be found in their natural state, having a 

 conical termination. 



27. The spiral vessels are found in dicotyledons in a layer 

 surrounding the pith called the medullary sheath, from which 

 they pass into the leaves and form a part of the ribs of those 

 organs. They are found in the sepals, petals, stamens, and pis- 

 tils, which are modifications of leaves. In monocotyledons they 

 occupy the central portion of every bundle of woody matter. 

 In acotyledons the true spiral is not found, but a modification 

 of it is found in all the Ferns, Equisetacese, and it varies in some 

 cases but very little from the true spii-al in the last-named family. 



28. The ofiice of the spiral vessels in the vegetable economy 

 is far from being determined. They took the name soon after 

 their discovery (by Grew, we believe) of Tracheae, from the sup- 

 position that they perform the same office in vegetables that the 

 organs of the same name perform in insects, but their true func 

 tion is yet unsettled. Many experiments have been adduced to 

 prove that they contain air only, and many also to prove that 

 their original function is to convey fluids to the recently developed 

 vegetable tissue. Both conjectures are perhaps true. In their 

 earliest stage they certainly contain fluids, and in the more ad- 

 vanced stages, it is equally certain they contain air, as may be 

 shown by cutting a stem under water, when bubbles will be 

 seen to form at the mouths of the spiral tubes. Bischoff has 

 obtained the air and analyzed it, and found it to contain six or 

 seven per cent, more of oxygen than common air. 



29. We think we may safely conclude that the true spiral 

 vessels perform diflerent functions in diflerent ages of the plant ; 

 and the more important of the two, and for which this tissue is 

 peculiarly adapted, is that of the earliest stage. We find it in 

 the earliest development of the plant. The extreme point of 

 formation, where the matter seems to be just passing from 

 mucilage into cellular substance, we find the spiral vessel. It 



27. Where are the spiral vessels found in dicotyledons? "Where in mo- 

 nocotyledons ? Are they found in acotyledons ? — 28. What is the otfice of 

 the spiral vessels? — 29. In wlmt stage of the plant do we find spiral ves- 

 sels ? Why produced here ? 



