6oo Hill . — The Histology of the 
I do not wish to lay too great stress on the two anomalous 
cases of callus-formation which have been brought forward, 
but I think they are not without value, as affording evidence 
of the way in which the callus may be formed in the one case 
from the cell-wall, and in the other from the cell-contents in 
connexion with the protoplasm. 
The Albuminous Cells, 
Before concluding this paper the peculiar connecting threads 
between the albuminous cells and the sieve-tubes, which were 
discussed at some length in a former paper 1 , must be briefly 
noticed again here. 
The youngest walls, in which threads could be seen, occurred 
within the limits of the boundary-cells of the phloem, and 
showed characters exactly similar to those of the pit-closing 
membranes of the very young sieve-tubes (Fig. 12, PI. XXXII; 
cf. also Fig. 8 ) ; but about the region of the boundary 
layer of the sieve-tubes changes are seen to commence which 
result in the development of the unequally divided pit-closing 
membranes so characteristic of the albuminous cells 2 3 , for on 
the sieve-tube side of the lamella the membrane thickens or 
swells to quite twice its original breadth, and the halves of the 
threads correspondingly increase in length, whilst on the side 
of the albuminous cell no great increase in size occurs ; the 
middle lamella is thus much nearer to the albuminous cell- 
cavity than to the lumen of the sieve-tube, and the appearance 
of the unequally divided membrane is given. The shorter 
portions of the threads retain their protoplasmic character 
throughout their existence, and stain darkly like the ordinary 
medullary ray cell-threads ; but the longer portions, on the 
sieve-tube side, pass through precisely similar phases of 
development to those which have been followed for the 
1 Hill, loc. cit., pp. 94, 118, &c., Figs. 8, 31, 36, 37. The figures of the sieve- 
areas of the albuminous cells in the stem are not very clear, owing to the callus 
having been omitted. 
3 Cf. figures in Strasburger’s ‘ Leitungsbahnen/ 1891, and ‘ Plasmaverbindungen,’ 
1901, 
