ENDOSPERM 19 
Here a well-defined plate is soon formed (PLATE 1, FIG. 7; PLATE 2, 
FIG. 30). These lower cells continue to divide and grow, pushing 
the mass into the lower cavity. The pressure of the liquid contents 
of this cavity opposes the encroaching mass and causes its dome- 
like form. Growth of the whole mass of endosperm continues, 
more rapidly near the lower margins, less rapidly in the central 
portion, until the residual cavity is surrounded except at the 
chalazal end where the base of the nucellus and the adjoining in- 
tegumentary tissue has been broken down. Soon after a definite 
plate of endosperm has been formed next to the residual cavity, 
there is developed a region of specially active tissue two or three 
cells above the lower surface of this plate (PLATE 2, FIG. 25). By 
the active multiplication and growth of the cells in this region the 
plate is forced downward into the residual cavity, and the mass 
of endosperm is increased. By the same action the cells of the 
endosperm bordering the residual cavity are subjected to lateral 
pressure between the restraining walls of the integuments until 
they become long, narrow and palisade like. During this time 
the large cells first formed have divided until they have surrounded 
the embryo with small, compact cells in 
every way similar to those composing 
the mass of the endosperm. 
When the endosperm mass is about 
one third formed there appears a 
peripheral layer of cells unlike those of 
the body of endosperm. By periclinal 
division of these an irregular layer of 
short flat cells is formed, varying abruptly 
from one to five cells in thickness. 
TEXT FIGS. 24-26 show consecutive steps 
in the development of this layer. These 
cells are ultimately filled with protein 
food stuff, probably aleurone. In the _ MSS 24-26. Three stages 
in the development of the pe- 
mature seed the endosperm cells, except dictions pubis teil Vasee 
the aleurone layer, are filled with starch of the endosperm. Fic. 26 was 
in the form of small simple and com- drawn from a section of almost 
pound grains. The process b egins in matureendosperm. All, X 52. 
the peripheral cells of the upper portion and extends downward 
