438 



THE MICROSCOPE. 



themselves throughout the entire length of the tube, those which were' 

 first deposited, and had then some slightly visible space between them, 

 become reinforced by others, or nourished by the contents of the vessel ; 

 so that space becomes obliterated, the fibre assuming a thread-like shape 

 with defined borders, and suflBciently large to allow of the transmission 

 of white light. When this action has progressed throughout the entire 

 vessel, the transparency is restored, and the entire mass of granules 

 disappears. The vessel having arrived at maturity, the liquid contents 

 are absorbed, as happens in the cells of the pith ; the vessel is then 

 empty. Probably its being seen in these difierent states, at one time 



fig. 211. 



. A portion of the leaf of Sphagnum, showing ducts, vascular tissue, and spiral fibrej 

 in the interior of its cells. 2. Porous cells obtained from the testa of Gourd-seed, 

 communicating with each other, and resembling ducts. 3. A transverse section of 

 Taxus iaccata (Yew), showing the woody fibre, i. Vertical section of the same, 

 exhibiting pores and spiral fibres. 5. A section from the stem of a coniferous 

 plant, with a transverse cutting magnified, and showing the zones of annual growth, 

 or annual rings. 



full and at others empty, may account for the discrepancies existing 

 among botanists as to the functions these vessels perform. The period 

 of growth at which the laying down of fibre commences, determines the 

 distance between the several coils ; for instance, when it is first formed 

 the coils are quite close, scarcely any perceptible trace of membrane 

 existing between them. In the annular vessel, the development of the 

 cell and the adherence of the granules to each other are conducted in 

 the same manner ; the deposit showing a tendency towards the spiral 

 direction, by the presence of a spire connecting two rings, or by a ring 



