SIEVE-TUBES 331 



In its young state before perforation of the septa takes place 

 every sieve-tube segment is provided with a more or less well-developed 

 peripheral layer of protoplasm and with a large nucleus, while the sap- 

 cavity is filled with a watery non-coagulable liquid. Later a number 

 of highly refractive homogeneous slimy globules of protein material 

 appear within the peripheral protoplasm. In the Cucurbitaceae these 

 slimy masses dissolve again before the sieve-tubes are fully developed, 

 but in other plants they are permanently retained. In the sieve-tubes of 

 Rohinia Pseudacada and other Leguminosae the slimy protein substance 

 is generally collected, according to Strasburger, into a large ellipsoidal 

 mass suspended near the middle of the tube by two fine threads, one of 

 which passes to each of the terminal sieve-plates. The peripheral 

 protoplasmic layer persists in the adult sieve-tube, and doubtless 

 extends over the walls of the sieve-pores, so that this peripheral proto- 

 plasm is continuous from segment to segment. The nuclei, on the 

 other hand, become disorganised and disappear, while the sieve-tube is 

 still in process of development. The peripheral protoplasm frequently 

 contains leucoplasts, with included starch-grains which assume a wine- 

 red coloration on treatment with iodine. The sap-cavities of fully- 

 formed sieve-tubes are filled with a clear, more or less concentrated 

 solution of protein compounds, which in the Cucurbitaceae has a slimy 

 consistency. As A. Fischer has shown, it is impossible to obtain a 

 correct idea of the distribution of the protein material in the tube, 

 unless the organ that is to be examined is immersed bodily in boiling 

 water before being cut into sections. When this precaution is observed, 

 it is found that the coagulated albuminoid slime is uniformly distributed 

 throughout each segment, and also extends through the sieve-pores. If, 

 however, the sections are prepared from living stems as had been 

 customary previous to Fischer's work the sieve-tubes become partially 

 emptied ; what remains of the contents forms a mass of slime in con- 

 tact with each sieve-plate in the shape either of a thin disc or a thick 

 plug (Schlauchkojif). In the majority of plants, as a matter of fact, 

 the concentration of the protein solution in the sieve-tubes is too low to 

 furnish any appreciable coagulum on boiling. 



Leaving aside the Thallophyta, which will be dealt with later on in a 

 separate section, it is found that sieve-tube-like rows of cells first 

 appear in the rudimentary leptome-groups of the central strand of the 

 stem in the most highly organised Mosses, namely, the Polytrichaceae. 

 The terminal enlargement of these cells, and the presence of a large 

 amount of protein material in their protoplasts, are both features which 

 recall the sieve-tube segments in the primary bundles of Angiosperms 

 Typical sieve-plates are not found in the Polytrichaceae, nor does any 

 formation of callus take place. It is nevertheless probable that 



