78 DUCTS SCHLEIDEN'S VIEWS. [BOOK i. 



vascular tissue open into each other at their points of contact, 

 and so become uninterrupted tubes of great length. 



The true history of ducts has given rise to much discussion, 

 and is still, in fact, sub lite ; the arguments of Mohl and 

 Schleiden, which represent the two opinions now current, may 

 be stated thus. (See Annals of Natural History, vols. vi. 

 and viii. for transactions of their original memoirs.) 



It will be evident from the illustrations already given in 

 the previous pages, that Mohl admits two kinds of secondary 

 deposits in the interior of tissue, a homogeneous or general for- 

 mation in some cases, whence the bothrenchym arises, and a 

 spiral formation in others. Schleiden, on the contrary, main- 

 tains that all deposits, of whatever nature, are spiral. This 

 author, in his famous memoir upon spiral formation, speaks 

 thus of the point at issue. He believes that the difference of 

 opinion in subordinate matters would disappear if the gradual 

 development of individual cells were kept more distinctly in 

 view, and especially if greater attention were paid to the expan- 

 sion of cells after the appearance of spiral deposits. " Thus," 

 he says, " in all my inquiries into the structure of the ligneous 

 body, I have never contented myself with comparing the 

 parts of different age of the same individual, but have con- 

 stantly, as far as the material was at my disposal, at the same 

 time pursued throughout a whole year the development of 

 the same annular ring, by repeated observations on the most 

 varied parts of the plants. Much instruction is likewise to 

 be obtained by watching the development of the spiroids in 

 large Monocotyledonous vascular bundles, as, for instance, in 

 Arundo Donax, where, however, we must not merely compare 

 on the same individual the younger with the older internodes, 

 but must examine homologous internodes on several indi- 

 viduals of different age. In this plant the spiroids in the 

 completely formed bundle, stood in a series radiating from 

 the axis to the circumference, and placed between two large 

 pitted vessels. Annular vessels, with the rings furthest from 

 one another, are nearest to the axis of the internode ; thence 

 towards the circumference the rings approach closer together, 

 then pass into broad threaded spiral vessels, and these last 



