FORMS AND SFSTEMS OF 7'ISSUE6'. 
89 
of them as a special form of coalescence of cells. They make their appearance in the 
young 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, 
enclosing thinner areolae. At a later period these latter appear to be actually perforated, 
while the thickening-ridges between them often swell up enormously. In this con- 
dition the septum, perforated by a number of pores, is termed a Sle've-plate ; it is 
usually broader than the diameter of the tube, which therefore appears dilated at its 
septa, the sieve-plates, and hence acquires a very characteristic form (Fig. 74). Sieve- 
plates of simpler structure are also usually formed in the side-walls where two sieve- 
tubes come into contact. In their early stage the sieve-tubes usually contain a tough 
albuminous mucilage very little affected by various solvents, which accumulates on 
both sides of the plate, and fills up the pores. The peculiar configuration of the 
sieve-plates, and the difficulty of obtaining longitudinal sections of them, render the 
observation of these characters extremely difficult ; 
this is especially the case with the perforation of 
the sieve-plates, which can, however, be proved by 
a method first employed by myself ^ It is suftl- 
cient to saturate thin longitudinal sections of the 
phloem with iodine-solution until the contents of 
the sieve-tubes begin to turn brown, and then to 
add concentrated sulphuric acid ; this dissolves the 
cell-walls and the substance of the sieve-plates, and 
nothing is left but the mucilaginous contents coloured 
a deep brown. The accumulations of protoplasm 
on each side of the sieve-plate are now seen to be 
united by slender threads of the same substance (Fig. 
74,/), which evidently previously filled the perfora- 
tions or Sieve-Pores ; and their continuity proves 
that the pores actually constituted a connection 
between two neighbouring tubes. Mohl gave to the 
sieve-tubes discovered by Hartig the term Latticed 
cells'^, since neither he nor subsequent observers 
were able to ascertain the actual perforation ; but 
Hanstein succeeded in determining it by means of 
Schulze's solution. It is not even yet by any means 
certain whether all the rows of cells in the phloem 
which have in recent times been called sieve-tubes 
have perforated sieve-plates, and are therefore the result of actual coalescence. The 
cells 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 pinnse 
of Cycadese, 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, I.e. 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 Ciicit7-bita; 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 whicli unite these accumu- 
lations and pass through the pores of the sieve- 
plates. 
^ Mohl, Bot. Zeit. 1855, p. 873. [Ann. des vSci. Nat. 1856, vol V. pp. 141 -159.]— Nägeli, Sitz- 
ungsber. der k. bayer. Akad. der Wissen. 1861. — Sachs, Flora, 1863, P- 68- — Hanstein, Die 
Milchsaftgefässe, Berlin 1864, P- 23 et seq. 
^ [Hartig termed the sieve-tubes Siebröhren, 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 ' ccllulec clathratcc,' ' cellules 
treillisees ' or ' grillagees,' 'latticed cells' or ' clathrate cells.'] 
^ Borscow, Jahrb. für wiss. Bot. vol. VIT. p, 348. 
