SPERMATOPHYTES—ANGIOSPERMS 
309 
divisions. In large sacs walls then begin to appear at the periphery 
and wall formation gradually advances toward the center until the 
entire sac is filled with tissue. Lilium, Peperomia, and Ranunnlus 
furnish examples of this type. 
An intermediate condition is seen in somewhat elongated embryo 
sacs of medium size, like those of Compositae. After a few free 
nuclear divisions, walls ap¬ 
pear simultaneously through¬ 
out the entire sac. Silphium 
laciniatum is particularly 
good. Akenes from which 
the corolla has just fallen will 
furnish material. 
The Embryo. —In most 
angiosperms the endosperm 
divides earlier than the fertil¬ 
ized egg and in some cases, 
like Asclepias and Casuarina , 
the free nuclear stage of the 
endosperm is completed and 
the cellular stage is well 
advanced before the first di¬ 
vision of the egg. In some 
forms, like the aroids, the 
embryo is massive and differ¬ 
entiation into dermatogen, 
periblem, and plerome comes 
comparatively late; while in 
others, like the Cruciferae, the 
differentiation occurs very 
early. Capsella is a standard example of the latter type (Fig. 114). 
The stages shown in Figure 114 A-F t will be found in pods about 3 mm. 
in length. These may be put directly into the fixing agent, but stages 
like G and H , which are found in pods about 5 mm. in length, should 
be trimmed as indicated in Figure 111#, before fixing. Formalin- 
alcohol-acetic acid is a good fixing agent; the special chromo-acetic- 
osmic acid, with 2 c.c. of acetic acid instead of 3 c.c., is also very 
good, and Delafield’s haematoxylin stains better after the chromic 
series. Cut 5 to 10 p thick and parallel to the flat face of the pod. 
Fig. 113 .—Lilium philadelphicum: photomicro¬ 
graph of section showing fertilization and also the 
triple fusion; from a preparation and negative by 
Dr. W. J. G. Land. X585. 
