SIEVE-TVBES OF ANGIOSPERMS. 



177 



length. Among these forms the tubes of the secondary bast of Ficus elastica and 

 Fagus sylvatica appear to be exceptions, since in them the lateral faces, turned 

 towards the periphery and middle of the stem, are covered thickly with sieve-plates, 

 which are only separated from one another by narrow fibre-like bands '■- These plates 

 on the lateral -walls are extremely delicate, and it cannot be determined whether they 

 really have pores, or are only portions of the wall having the latticed appearance of 

 sieve-plates. 



In the tubes with ladder-like terminal faces the series of plates is continued, 

 usually quite gradually, from these to the neighbouring lateral surfaces, and especially 

 on to the radial ones : in the bast of the Dicotyledons the arrangement is always such 

 that the plates on the lateral surfaces are smaller and wider apart than on those 

 which are terminal. 



The contents of the fresh intact sieve-tube as it lies in water appear as trans- 



FIGS 72 71 —Two large sieve-tubes of Lagonaria yulgaris in longitudinal section, where two members join, 

 after action of alcohol and iodine solution. In Fig. 72 i' is the non-callous widely-perforated horizontal plate, 

 as seen in longitudinal section ; >• the contracted sac-like contents, with the dense aggregation of slime. Pro- 

 cesses from this traverse the pores on the left side ; on the right side M they have been torn out of these in 

 cutting the section. In fig. 73 the transverse plate (,r) is oblique ; thus the half of it present m the prepara- 

 tion is seen in section, and obliquely in surface-view. The coagulated slimy contents have been quite sepa- 

 rated from it in cutting the section, and the plate, which rested on it, lias been so turned that its .hole sur- 

 face, which before abutted on the intact sieve-plate, is turned towards the observer. The processes, which 

 before fitted into the pores, appear upon it as rings (375). 



parent as water. More exact investigation shows that the wall of each member of the 

 tube is invested by a continuous thin layer of almost homogeneous slimy substance 

 resembling protoplasm. This layer surrounds a central watery fluid, to which must be 

 ascribed the alkaline reaction '■= characteristic of the contents of bundles of sieve-lubes, 

 at all events in Cucurbita. At one end, or more rarely at both ends, of the member 

 it encloses an apparently dense lustrous aggregatiori of shmy substance, which hes 

 upon the sieve-plate, either as a thin lamella, or as a plug of relatively considerable 



' Von Mohl, I.e. ; Dippel, Mikroskop, p. 255. 

 ' Compare Sachs, Botan. Zeitg. 1862, p.- 257. 



N 



