Johnson . — On Arceuthobium Oxycedri . 153 
pericarp had passed through most of the changes seen in it in 
a ripe fruit; the viscid cells were comparatively well-developed ; 
and yet between the inner surface of the endocarp, of which 
the viscid cells were the outer surface, and the shrivelled 
ovarian papilla (‘nucellus’ of Baillon) there was a large gap 
'of the same nature as that in the ripe fruit between the 
endocarp and the endosperm. It was of interest to find all 
the specialised accessory modifications in the fruit, while the 
essential parts, embryo and endosperm, were quite absent. 
Returning to the ripe fruit, the mesocarp consists of two strata, 
an inner one composed of thin-walled cells pressed completely 
out of shape, and an outer one, which, between the points a and 
b in Fig. 9, consists of five or six layers of thick-walled pitted 
parenchymatous cells. The walls are lignified and the pits 
wide (Fig. 11). The change from this sclerotic tissue to thin- 
walled cells above b and below a in Fig. 9 is very abrupt. 
At a , Fig. 11, represented on the surface of the fruit by a 
circular horizontal groove (the place of articulation of the 
fruit with its peduncle), there is, as seen in a longitudinal 
section, a horizontal plate of extremely thin-walled cells, eight 
to ten tiers high, and formed very probably by the meri- 
stematic activity of a single layer of cells. Where this meri- 
stematic tissue abuts against the vascular bundles the xylem 
vessels atrophy. Between its uppermost layer and the base 
of the endosperm the lower part of the ‘ endocarp/ some five 
layers thick, is situated. At dehiscence of the fruit this zone 
of meristematic tissue is torn in two horizontally, the vascular 
bundles being also transversely cleft. (Cp. Fall of Leaf 1 .) 
Taking these structural facts into consideration, it seems to 
me an anatomical explanation of the mechanism of dehiscence 
of the fruit may be given. Before fertilisation the unoccupied 
part of the ovary is very small, the ovarian papilla is almost in 
contact with the wall of the ovary. This is not less so after 
fertilisation. The formation of the bulky endosperm begins 
simultaneously with intercalary divisions of the pericarp. 
1 Strasburger, Eot. Pract., 1887, p. 223. 
M 
