SIEVE-TUBES OF GVMNOSPERMS. 



179 



sieve-pores, they must often also include starch-grains. This requires more exact 

 proof. The starch-grains are doubtless densely crowded on the sieve-plate, and 

 especially so on the pores. But they cannot so easily and generally enter and pass 

 through the pores, since they are often larger than these : for instance, in Vitis, at 

 the period of most active vegetation, they are on the average twice as broad as the 

 pores. (Fig. 74, a) 



The structure described above is found in fresh intact sieve-tubes. But it 

 appears much more plainly after the action of reagents. On treatment with alcohol 

 the peripheral layer, resembling protoplasm, immediately coagulates along the sides of 

 the members, separates from the membrane, and contracts to a relatively thin, folded, 

 but still closed sac, which occupies the middle of the member (Figs. 68, 72, 74). 

 On the face which is in contact with the sieve-plate, and which is attached by the 

 processes in the pores, the sac retains its original width, or at least that of the 

 perforated part of the wall : it thus widens more or less rapidly in a conical manner 

 opposite these faces : the processes which enter the pores alter their form and position 

 but little or not at all. 



Iodine preparations produce the same changes in form, and colour the whole peri- 

 pheral layer and the terminal aggregations of slime deep yellow to yellowish brown ; 

 the starch-grains violet ^ : this coloration appears much more quickly in the parts in 

 question than in the callus-masses, so that by this means these two parts may easily be 

 distinguished from one another : this renders the understanding of the structure more 

 easy, especially in slightly-thickened and widely-porous sieve-plates, inasmuch as in 

 this case the figures of the sieve-plates on the one hand, and on the other of the 

 plates of slime (with their processes), which cover the sieve-plates, are necessarily 

 similar in the surface-view of the plate, and are often difficult to distinguish at first 

 sight. According to the above behaviour with alcohol, and preparations of iodine, 

 and other known chemical reactions^, the slimy contents of the sieve-tubes, i.e. both 

 the lateral peripheral layer and the terminal aggregations, consist in the main of an 

 albuminoid substance similar to protoplasm. It is doubtful whether it should really 

 be termed protoplasm, less because of the slight differences between the iodine 

 reaction of the slime and of the protoplasm of the surrounding tissue in Cucurbita ^, 

 than because protoplasm is a body which is characterised not only by its material 

 composition, but also by a definite organisation or structure, which e.xpresses itself in 

 protoplasmic movements, differentiation of nuclei, &c., and since phenomena such as 

 the above have not been observed in the contents of sieve -tubes. 



Sect. 44. In the Gymnosperms and Fern-like plants'^ tubes are found, in similar 

 places to the sieve-tubes of the Angiosperms, which, from their great similarity to 

 these, are doubtless rightly included under the same term, but differ in certain 

 points from them, and especially in the character of their contents. 



The sieve-tubes of those Gymnosperms which have been investigated — e. g. Larix, 

 Abies pectinata, Juniperus, Sequoja gigantea, Salisburia, Ephedra, Gnetum, Encepha- 

 lartos — are similar in form and average size of the members to those commonly 

 found in the bast of ligneous Dicotyledons, which have the ends of the members 



' Compare Bxiosi, I.e. * Compare Sachs, Flora, 1863, p. 38. ^ Nageli, /. c. p. 16. 



* [See Janczewski, I.e.; also Russow, I.e., and Strasburger, Zellhaute, p. 57.] 



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