SIEVE-TUBES. 



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and in which the canals are only indicated by transverse striae, and by funnel-shaped 

 depressions of the surface : in others even these indications are not noticeable 

 (Fig. 67, 76). 



The callous plate always consists of three lamellae, one central, and two applied 

 to this laterally, one on each side : each of these belongs to one of the members of 

 the tube. The middle lamella is the original cellulose sieve. The lamellae of callus 

 are in the fresh condition homogeneous, colourless, apparently soft, and having by 

 transmitted light the peculiar bluish lustre of gelatinous membranes : they are 

 coloured yellow by solution of iodine in potassium iodide, and by Schultze's solution 

 a deep brownish-yellow : in sulphuric acid they swell till 

 their oudine is completely lost. A similar swelling results 

 from the action of alkalies, especially solution of potash, 

 and of Schultze's mixture. By these reagents the callus 

 mass may be completely removed from the persistent cellu- 

 lose sieve. 



At the margin of the sieve-plate, next the adjoining 

 mem.brane, the callus mass stops rather abruptly. 



From all these phenomena it is concluded that the 

 callus mass is formed by apposition upon the original cellu- 

 lose sieve. The conditions of its appearance and its 

 physiological significance require further investigation : ac- 

 cording to some few experiments on Cucurbita and Lage- 

 naria, the callous thickening seems in these cases to appear 

 and increase with the age of the sieve-tubes, and in the 

 first-formed (peripheral) tubes of a vascular bundle it seems to 

 advance very quickly till the sieve is entirely closed. In many 

 plants sieve-tubes may be found side by side without callus, 

 and with callus in the most different stages, e. g. Lagenaria : 

 in others, e.g. in the bast of Quillaja, only callous sieves 

 are known, but they are always open. In the bast of many 

 ligneous plants — Vitis, Tilia — I find all the sieve-plates 

 completely closed by callus in the winter time; in the height 

 of summer they are open and not callous. (Comp. Figs. 

 69, 74, and 76.) 



The sieve-plates are always placed on the terminal 

 faces of the cylindrical tubes. If these faces are horizontal, 

 -or only slightly inclined, each has throughout the properties 

 of one sieve-plate, which may be termed a simple transverse 

 plate : this is so in all the above-named cases of the original 

 vascular bundles and primary bast of Angiosperms (comp. 

 Figs. 65-67), in the slightly inclined terminal faces of Ca- 

 lamus, in the secondary bast of Fagus and Quillaja. 



On strongly-inclined terminal faces the sieve-plates are arranged like the scalari- 

 form openings of vessels in series one above another, and are, like these, separated 

 from one another by narrow bands of membrane : they usually form a single series, 

 rarely they form here and there several irregular rows. Examples of this are supplied 



H^i^iiilliiliiiV 



Fig. 68. — Cucurbita Pepo ; mature 

 internode, longitudinal section, parts 

 of two sieve-tubes, with large callous 

 plates at their terminal faces. The 

 contents of both contracted by al- 

 cohol, but attached to the plates, and 

 passing through the pores from one 

 member into the other. 



