48 Gibbs —Notes on the Development and S true hi re of 
also possibly correlated with the differentiation of the c aerenchyma * in 
relation to the supply of water and oxygen. 
We have seen that the integuments consist each of two layers which, 
in the case of the inner integument are undifferentiated, the cells of the 
apical portion merely increasing in size where they project beyond the 
outer integument. This part is subsequently used up during the passage 
of the pollen-tube. Over the periphery of the ovule they lose their cell 
contents, and become so stretched as growth goes on that the dividing walls 
disappear, leaving apparently one layer only. 
The outer integument is composed of two differentiated layers, layer 
1 being purely protective, increasing its area by the papillar outgrowth of 
the cells forming it and its mechanical function by the cuticularization of 
the cell-walls. It is possible that layer 2, the cells of which remain active 
and functional till maturity by dividing to keep pace with its growth, may 
act as a water jacket, forming a sort of transitory water storage tissue for 
the growing ovule. This hypothesis is strengthened by a comparison with 
the mode of its development in some of the Sperguleae. In these plants 
a proliferation of the cells of the layer under discussion results in a local 
hypertrophy, ultimately forming a wing which extends round the ovule in 
the vertical plane of the embryo, but in its earlier stages is very suggestive 
of a transitory water storage function. 
The nucellus is differentiated into two regions, viz. : i. The peripheral 
layers, which are available for solution by the cytoplasm of the embryo-sac 
to provide for increase in size, and which when the latter obtains its 
maximum growth are gradually reduced to one layer, which persists till 
the germination of the seed and even in the discarded seed coat. ii. The 
axile rows which receive and distribute the supplies of food material from 
the chalaza in their basal portion and elaborate the starch reserves or 
perisperm in the upper cells of these rows, the cells increasing greatly in 
size as the starch is laid down. 
2. The manner in which the food supplies are made available. That this 
occurs in the first place through the agency of the suspensor is suggested 
by the remarkable form assumed by the latter owing to the great differentia- 
tion of the basal cell of the filament. The early differentiation of that cell 
points to the same conclusion since it assumes its final shape even before 
the first division of the oospore (PI. V, Fig. 9, oospi). 
The persistence of the pollen-tube, and the characteristic plug formed 
by it on the apex of the embryo-sac, may also be interpreted as cor- 
roborative evidence for the activity of the suspensor, as the channel formed 
by the pollen-tube in its passage through the nucellus is kept open, thus 
increasing the area available for solution. In Stellaria media , where the 
tube is not persistent and does not form a plug, the plant has overcome the 
difficulty in reaching the apical nucellar tissue by sending an haustorium 
