the Grain of Wheat (Triticum vulgare). 133 
slightly in the later stages, while most of the sub-aleuronic cells retain 
dense nuclei to the end. This may indicate that the nuclei which stain so 
slightly are to all intents and purposes dead, while the others still retain 
at least some degree of vital energy ; but such a suggestion can only be 
brought forward with the utmost reserve, because at the time that the 
nuclear networks are forming the wheat grains are undergoing considerable 
desiccation, and so the pressure on the tissues is probably increased by 
shrinkage. It may easily happen that the desiccation also affects the 
nuclei, and the loss of staining power may be due to mere physical changes 
partly caused by the withdrawal of water. 
A similar phenomenon, due to the squeezing action of starch grains, 
has been noticed in barley by Brown and Escombe (16), who describe the 
‘ senescence ’ and disintegration of the nuclei of the endosperm as maturity 
approaches. According to these authors, deformation of the nucleus begins 
in the cells underlying the sub-aleuronic layer, and rapidly advances 
centripetally till nearly the whole of the endosperm is involved, the 
deformation being followed by complete disintegration of the nucleus. 
In the later stages of ripening, the cells of the sub-aleuronic layer also 
become involved, the very last cells to undergo this ‘ nuclear senescence ’ 
being those of this layer in the ventral folds on either side of the 
furrow. This may be paralleled in the wheat, in which the nuclei in 
this same position are the last to lose their nucleoli and become dense 
in structure, but the complete disintegration observed in barley does 
not occur in Tritictim , as the fine networks can be made out to the end. 
The way in which the disintegration proceeds throughout the length 
of the grain is not touched on by Brown and Escombe, so it is possible 
that a more complete examination would reveal some nuclei remaining at 
the further end of the grain which would compare with the dense sub- 
aleuronic nuclei of the wheat. 
(d) Development of the Embyro. 
Strasburger (13) gives a very complete account of the structure of the 
embryo in the ripe seed, but does not enter into the developmental history. 
The egg cell after fertilization enters on a short period of rest till about the 
fifth day after pollination, when the first division occurs. Koernicke 
mentions a three-celled stage as being much in evidence, but unfortunately 
the material does not show it. 
The youngest segmented embryo observed consisted of a mass of four 
or five cells apparently attached to another cell nearer the micropyle, pre- 
sumably the suspensor, but the preparations were not sufficiently clear to 
be certain of this point. 
During the next eight or nine days cell-division proceeds actively, and 
the young embryo develops into a mass of cells, rapidly increasing in size. 
