FORMS AND SFSTEMS OF TISSUES. 



89 



)f them as a special form of coalescence of cells. They make their appearance in the 

 roung phloem in the form of long tubes arranged in rows, with thin walls and trans- 

 verse or oblique septa, on which a net-work of thickening-ridges is soon observed 

 tnclosing thinner areolae. At a later period these latter appear to be actually perforated 



^hile the thickening-ridges between them often swell up enormously. In this con- 

 htion the septum, perforated by a number of pores, is termed a Sie've-plate ; it is 



sually broader than the diameter of the tube, which therefore appears dilated at its 



?pta, the sieve-plates, and hence acquires a very characteristic form (Fig. 74). Sieve- 

 )lates of simpler structure are also usually formed in the side- walls where two sieve- 

 ^ubes come into contact. In their early stage the sieve-tubes usually contain a tough 

 llbuminous mucilage very little affected by various solvents, which accumulates on 



)th sides of the plate, and fills up the pores. The peculiar configuration of the 

 peve-plates, and the difficulty of obtaining longitudinal sections of them, render the 

 observation of these characters extremely difficult ; 



lis is especially the case with the perforation of 

 ^he sieve-plates, which can, however, be proved by 



method first employed by myself ^ It is suffi- 

 :ient to saturate thin longitudinal sections of the 

 )hloem with iodine-solution until the contents of 

 the sieve-tubes begin to turn brown, and then to 

 idd concentrated sulphuric acid ; this dissolves the 

 :ell-walls and the substance of the sieve-plates, and 

 lothing is left but the mucilaginous contents coloured 



deep brown. The accumulations of protoplasm 

 )n each side of the sieve-plate are now seen to be 

 inited by slender threads of the same substance (Fig. 

 '4, p), which evidently previously filled the perfora- 

 tions or Sie've-Pores ; and their continuity proves 

 that the pores actually constituted a connection 

 )etween two neighbouring tubes. Mohl gave to the 

 sieve-tubes discovered by Hartig the term Latticed 

 veils'^, since neither he nor subsequent observers 

 ^ere able to ascertain the actual perforation; but 



[anstein succeeded in determining it by means of 

 Jchulze's solution. It is not even yet by any means 

 certain whether all the rows of cells in the phloem 

 7hich have in recent times been called sieve-tubes 



lave perforated sieve-plates, and are therefore the result of actual coalescence. The 

 :ells of the parenchymatous fundamental tissue also not unfrequently exhibit a sieve- 

 plate-like structure on their walls (see Fig. 21, p. 24), as, for example, in the pinnae 

 of Cycadeae, the bark of Ceropegia aphylla (Asclepiadeae), &c. ; but with respect to the 

 latter Borscow unhesitatingly asserts^ that they are not perforated, the pores being 

 still closed by thin membranes. It is an interesting fact that the laticiferous cells of 

 Ceropegia, as well as (according to David, /. c. p. 57) those of Euphorbiaceae, are connected 



Fig. 74. — Places where sieve-tubes unite, 

 showing; the perforation of the septa after solu- 

 tion of the cell-wall by sulphuric acid. A and B 

 from the petiole of Ciicicrbita ; C from the 

 stem of the dahlia. In A the cell-wall h h' is not 

 yet completely absorbed; s' the protoplasmic 

 mucilage, o and 71 accumulation of it on the upper 

 and under side of the septum (sieve-plate) ; / the 

 threads of protoplasm which unite these accumu- 

 lations and pass through the pores of the sieve- 

 plates. 



^ Mohl, Bot. Zeit. 1855, p. 873, [Ann. des Sci. Nat. 1856, vol. V. pp. 141-159.] — Nageli, Sitz- 

 ungsber. der k. bayer. Akad. der Wissen. 1861, — Sachs, Flora, 1863, P- 68, — Hanstein, Die 

 Milchsaftgefasse, Berlin 1864, P- 23 et &eq. 



^ [Hartig termed the sieve-tubes Siebrohren, which has been rendered ' cribriform vessels ' by 

 some English, and ' tubes cribreux ' by French writers. Mohl preferred, for the reason stated in the 

 text, to call them ' Gitterzellen,' which has been variously rendered ' cellulse clathratoe,' ' cellules 

 treillisees ' or ' grillagees,' ' latticed cells ' or ' clathrate cells.'] 



^ Borscow, Jahrb. flir wiss, Bot. vol. VII, p. 348. 



