i8o 



SIEVE-TUBES. 



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bevelled like a chisel. They may, like these, attain a considerable width, e.g. 0.030™'" 

 in the secondary bast of old roots of Abies pectinata. The oblique terminal faces 

 are directed, both in the stem and in the roots, towards the radial planes (medullary 

 rays). Sieve-plates are distributed uniformly in one or two longitudinal rows over 

 the terminal faces, and the whole remainder of the radial lateral face. They form 

 roundish spots, separated by high intervening portions, or are rarely elongated 

 transversely, and separated by narrow ring-like bands : these spots are coarsely 



latticed, while in the cavities of the coarse lattice the very delicate 

 sieve-structure is seen (Figs. 77, 78). Considering their close 

 similarity to like parts of Dicotyledonous plants, there is no 

 reason to doubt that the channel through the narrow sieve-pores 

 is open. But this has not been directly proved, and the proof 

 has hitherto been impossible, since the tubes in the plants in 

 question are filled almost exclusively with watery fluid. The 

 masses of starch-containing slime, giving the reactions of pro- 

 toplasm, which in the Dicotyledons send their processes through 

 the • sie.ve-pores, have not yet been discovered in the plants in 

 question : on the walls of the tubes there are attached internally 

 some few very small granules, which turn jellow with iodine. 

 The nature of the materials composing the fluid contents re- 

 quires further investigation. Further, I was unable to find a 

 formation of callus, with the exception of a doubtful case in 

 the root of Abies pectinata. 



Among the Ferns a number of plants have relatively large 

 and wide vascular elements, and among these such as are, from 

 their position (comp. Chap. VIII), and their structure, to be 

 enumerated among sieve-tubes. This is the case in many 

 Polypodiacese, e.g. Pteris aquilina (Fig. 79), Marsiliacese (Mar- 

 silia Drummondi and its allies), Cyatheaceae, Osmundacese, 

 Ophioglossese, according to Dippel in the Equiseta, and at least 

 the larger Lycopodia ^ 



In the Equiseta and the Ophioglossese they consist, ac- 

 cording to Dippel and Russow, of tabular prismatic members, 

 which stand one upon another in longitudinal ro\vs with hori- 

 zontal; callous, sieve-like, transverse walls. The lateral walls 

 have no sieve-pits. 

 In the other cases cited, the members of the tubes are fitted one on another with 

 pointed ends (in Marsilia also with horizontal ones), and have sieve-plates both on the 

 latter, and also on the whole of those lateral surfaces which are contiguous with similar 

 elements. These are usually elongated transversely, forming, according to the width 

 of the surface of wall, one or several rows : in these rows they are either crowded 

 closely, and separated only by narrow bands of wall (Fig. 79, B), or they are at a 

 considerable, and then usually a variable distance from one another. The sieve- 



Ml 



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Fig. 77. — Sequojagigantea. 

 End of a member of a sieve- 

 tube, from the bast of an old 

 stem, presenting^ the radial 

 face. The uppermost (radial) 

 wall is shaded. On it are small 

 grouped sieve-plates with 

 very fine pores, indicated 

 only here and there by points 

 (375). 



^ Dippel, Eericht d. 39. Versamml. deiitscher Naturforscher zu Giessen, 1864, p. 146, Taf. IV. 

 — Idem, D. Mikroskop, pp, 195, 203. — Russow, Vergl. Unters. pp. 5, loi, 118, 129, 142. 



