Vegetable Physiology. 83 



ijj~quadruple. These vessels are extremely delicate, their 

 diameter averaging the 1000th of an inch. The bark, wood 

 or root seldom contains them, but in the stem and leaf stalk 

 they are often found, and they form almost the whole of a 

 peculiar part which surrounds the pith of some plants, called 

 the medullary sheath. They may be detected in the stringy parts 

 of the stem of the Asparagus, more readily, than perhaps in 

 any other situation, by separating the strings by boiling, and 

 placing one of them upon a piece of glass under the micro- 

 scope, and then further dividing it lengthwise with the point of 

 a needle. These vessels are very similar in their construction 

 to the air-tubes of insects. The principal difference between 

 the two is, that those of the insect branch off into a set of con- 

 tinuous tubes, all having a direct communication with each 

 other, while those of the plant are parallel, and their ends are 

 closed. All the varieties of vessels not furnished with an elas- 

 tic spiral filament, are named Ducts. These are considered 

 as only elongated cellules, and seem to be formed by the 

 breaking down of the partition between the cells, which as we 

 have seen compose cellular tissue, so that a continuous tube 

 is produced. Sometimes the remains of these partitions are 

 discovered in them. The variety of duct which approaches 

 most nearly to simple cellular tissue is the dotted duct, Figure 

 6, g. This is formed by a membranous tube in which appear 

 to be the remains of a spiral fibre broken into small fragments. 

 Some observers, however, consider that this duct is membra- 

 nous only, and that what appear to be portions of fibre, are 

 only an interior deposite. The dotted duct is the largest vessel 

 discovered in the vegetable fabric, many of them being visi- 

 ble to the naked eye, and of sufficient diameter to admit a 

 hair. If the stem of a grape-vine be cut across, their mouths 

 can be distinctly seen, and the sap will flow from them freely. 

 Their office is the important one of conducting the sap through 

 the stem and branches to the leaves. Another kind of duct, 

 which more nearly resembles the spiral vessel, is the annular 

 duct, Figure 6, e. This shows clear traces of a spiral fibre 

 running within its membrane, sometimes broken so as to 

 form a complete ring, so that it would seem as if the mem- 

 brane had grown faster than the fibre, which, losing its elasticity, 



