IN VEGETABLE BIOLOGY. 377 
It is now generally agreed that the palisade-tissue is the chief 
locus of assimilation, the products of which pass, by the meso- 
phyll, to the bundle-sheath, and so to the bast*. I venture to 
doubt, however, whether this be the whole truth of the matter. 
The leaf-cells of Bryonia dioica contain a substance (the so-called 
“soluble starch ") blueing or purpling with iodine, and pre- 
senting certain of the reactions of tannin; it is regarded by 
Dufour t as a carbohydrate, by Kraus as tannin. Whatever its 
nature, this body is undoubtedly a product of assimilation ; it is 
therefore an easy matter, after causing its disappearance by the 
aid of darkness, to ascertain the precise direction’ taken by it, 
since with weak iodine it gives a characteristic blue reaction. 
The epidermal cells lining the lower side of the leaf contain 
numerous well-coloured chlorophyll grains in which starch can be 
detected ; the upper-layer cells are without chlorophyll§. The 
substance in question is discoverable, by the indicated method, 
not only in the mesophyll and above all in the palisade-tissue, 
but also in considerable quantity in both upper and lower epi- 
dermis, and likewise in the basal cell and one or two of the guc- 
ceeding cells of the hairs. When a stronger solution of iodine 
is used, a markedly deeper purple is seen in the somewhat 
elongated epidermal cells overlying the vascular bundles than in 
the other cells. This seems to show that in addition to the 
downward passage of assimilated matter already adverted to, there 
are two subsidiary overflow streams—an upward one from the 
palisade region, and a lower one from the mesophyll into the 
upper and lower epidermis respectively, which latter tissues and 
the hairs would appear to be important reservoirs of assimilated 
material; aud the accumulation of this in the cells overlying the 
vascular bundles indicates its transport laterally in the epidermis. 
If this be so, Haberlandt’s case is strengthened, as a reason is 
now, upon his doctrine, furnished for the absence of ehlorophyll 
from the upper transverse wall of the palisade-cells, t. e. the wall 
abutting on the epidermis. N evertheless, I am unable to agree 
with this author in regarding the position occupied by chlorophyll 
* On this point see Haberlandt, in Prings. Jahrb. f. wiss. Bot. xiii. 1882 ; 
Strasburger, Practicum, Pensum XVII. ; Schimper, Bot. Zeitung, 1885. 
t Bull. Soc. Vaud. d. Se. Nat. 1886 (see Bot. Zeitung, 1886, p. 869). " 
ł Abhandl. naturf. Gesell. Halle, 1885; briefly abstracted in Journ. s 
Micros. Soc., Feb. 1886. 
$ Journ. of Bot., Dec. 1887. 
