SIEVE.TVBES OF CRYPTOGAMS. 



I»I 



on one 



Fig. 78. — Encephalartos 

 pungens. Bast of an old 

 stem. Part of the radial 

 wall of a sieve-tube (375). 



permeability of the 



plates are not callous. Their pores, as far as they can be recognised, are very 

 narrow and round ; and in Marsilia, according to- Russow, very numerous 

 plate; in the cases investigated by me (Pteris aquilina, Cyathea, 

 Alsophila spec, Osmunda) they are less numerous, and relatively 

 far distant from one another. , The wall of the tubes is thin at 

 the sieve-plates ; the rest of it is strongly thickened, stratified, 

 and soft, and apparently swells in water. These tubes contain 

 a quantity of watery fluid, and a thin peripheral layer, coloured 

 yellow by iodine, which contains throughout, and especially at 

 the ends of the members, and on the lateral sieve-plates, 

 numerous very small granules which adhere closely to the wall. 

 In dried-up tubes the ends are also found filled with a homo- 

 geneous brown mass. These granules are not starch: they 

 turn a deep yellow with preparations of iodine : maceration in 

 dilute solution of potash destroys them only partially even after 

 many days. Their dense aggregation and their tenacious hold 

 on the sieve-plates usually prevents a clear decision on the 

 pores : but I believe that I -have clearly 

 seen in thin longitudinal sections in 

 Pteris aquilina that the granules of con- 

 tiguous sieve-tubes are connected by 

 thin filamentous processes which tra- 

 verse the transverse pores (Fig. 79, c). 



The tubes are not inferior in width 

 to the medium and thicker tubes of 

 the Gymnosperms. The length of the. 

 single members is considerable, in the 

 cases investigated (Pteris aquilina, Cy- 

 atheacese) it is 1-3™™. In the Mar- 

 silias they attain, according to Russow, 

 the length of one whole internode, that 

 is, of several centimetres, a statement 

 which may have its origin in the ease 

 with which the ends of men;bers may 

 be missed in tubes prepared free by 



maceration. 



In the larger indigenpup Lygopo,- 



dia (L. clavatum, annotinu,m) ther?. 



occur in the vascular bundles of the 



stehi organs which, in their position 



and width, have great similarity to the 



members of sieve-tubes of the above 



Ferns. They are prismatic and elon- 

 gated, so that their pointed ends can 



seldom be seen in sections. Their 



contents correspond also to those of 



Fig. 79.— Pteris aquilina, Rhizome. A end of a member of a sieve- 

 tube, isolated by maceration (142)4 B part of a thin longitudinal sec- 

 tion. This has approximately halved two sieve-tubes, si and J2, 

 which are so drawn, that the plane of section faces the observer, anti 

 the uninjured side hes behind. The latter is represented as ligliter, 

 while that which lies in the plane of section is darker. Ji abuts to the 

 right on parenchymatous cells which have been cut through, to the left 

 on j^; at the back again, its broad surface covered with sieve.plates, 

 is contiguous with a sieve-tube, while to the left and at the back 

 it abuts with a smooth wall on parenchymatous cells. J2 borders with 

 its whole smooth-walled posterior side on parenciiyma (the nucleus is 

 indicated m two of the cells), right and left on sieve-tubes ; r, c sections 

 of walls bearing sieve-pits (375). 



