178 SIEVE-TUBES. 



height. Usually this aggregation of slime is found at one end only of the member, 

 and in that case, in Cucurbita according to Nageli, in ^ of the instances (taking the 

 whole plant into consideration) it is at the upper end, that is, on the under surface of 

 the sieve-plate. In very many cases numerous very small grains of starch are im- 

 bedded in the slime, and especially in the terminal aggregations of it V ■ Briosi found 

 these in the stems and leaf-stalks of 129 out of 146 species investigated. At the 

 sieve-plates the slimy contents are continuous through the pores from one member 

 of the tube into the adjoining one. It may be seen, especially in callous plates, and 

 •when coloured yellow with iodine, filling all the pores and demonstrating, like a 

 natural injection, the open communication through them (Figs. 68, 74). Where 

 however the size of the parts makes an exact investigation possible, it may be seen 

 that it does not pass as a homogeneous mass equally from one member into the 



' FIGS. 74, 75. — Vitis vinifera, Hast ; from the same branch as tig. 69, prepared on the same day (600). 



FIG, 74. — Tangential section through the ladder-like limiting surface of two members of a sieve-tube A and B, In A the dense 

 plug of slime, contracted by alcohol, sending blunt processes through all the sieve-pores into B ; a ^ grain of starch. 



Fig. 75. — Kadial section, after action of absolute alcohol, destruction of th e starch by brief action of strong solution of potash 

 {the latter has caused a slight swelling of the membrane), and subsequent washing out of the potash, and treatment with iodine. 

 Fart of a ladder-like wall in surface view : beneath it the shrunken slimy contents of an adjoining member, which sends capitate 

 processes — upwards as the preparation lies — through the pores into the other member. 



FIG. 76. — Bast from a branch several years old, and 1*5 <="> thick, of the same plant in winter. Callous closed wall between two 

 members of a sieve-tube, tangential section {400). 



Other, but that the peripheral layer of the one member sends processes into the 

 pores, which they fill, and end blind at the limit of the adjoining member : the 

 processes either end simply at the surface of the sieve-plate, or are more or less 

 swollen, and rise above it into the cavity of the adjoining member, while at the point 

 of transit through the plate they fit into corresponding holes in the peripheral layer 

 of the member, which they enter (Figs. 72-75). As far as is at present known the 

 processes always extend on one sieve-plate to one side only, thus from the member 

 a to b, and not also conversely : further, they extend from the surface on which there is 

 the larger collection of slime to the other. They are in their turn also filled with slimy 

 contents. According to Briosi's statement, that the starch-grains often stick in the 



' Briosi, Ueber allgemeines Vorkommen von Starke in den Siebrohien, Botan. Zeitg. 1872, p. 

 305. — Compare also Sachs, Exp, Physiol, p. 383, &c. 



