28 VASCULAB TISSUE. 



well with the one named, as the Asparagus, Strawberry, 

 Currant, Dogwood, ike. In the above cases the vessels are 

 not seen in their natural state, since they are seen uncoiled. 

 In the stem, the fibre that we see uncoiled when pulled 

 apart, forms a complete tube by its edges coming in contact 

 in coiling. But if either the Poke-weed or Asparagus be 

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

 a conical termination, as exhibited in fig. 13, b. 



29. The spiral vessels are found in exogens in a layer sur- 

 rounding 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 

 pistils, which are modifications of leaves. In endogens they 

 occupy the central portion of every bundle of woody matter. 

 In acrogens the true spiral is not found, but a modification of 

 it is found in all the Ferns, Equisetacse, and it varies in some 

 cases but very little from the true spiral in the last named fa- 

 mily. More extended observations are needed to settle this 

 question. 



30. The office ot^ the spiral vessels in the vegetable econc 

 my is far from being determined. They derived the name 

 soon after their discovery, (by Grew, I believe) of Tracheae, 

 from the supposition, that they perform the same office in ve- 

 getables, that the organs of the same name perform in insects, 

 but their true function 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 per- 

 haps are true. In their earliest stage they certainly contain 

 fluids, and in the more advanced stages, it is equally certain 

 they contain air, as may be shown by cutting a stem under 

 water, and 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. 



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

 vessels perform different functions in different 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 to organic substance, we find the 

 spiral vessel. It seems to me that in this case we find an 

 adaptation peculiarly fitted to accomplish a given end, and it 

 would require not a very great stretch of imagination in con* 



