SPERMATOPHYTES— ANGIOSPERMS 



369 



lar stage is well advanced before the first division of the egg. In some 

 forms, like the aroids, the embryo is massive, and differentiation into 

 dermatogen, periblem, and plerome comes comparatively late; while in 

 others, like the Cruciferae, the differentiation occurs very early. Cap- 

 sella is a standard example of the latter type (Fig. 136). The stages 

 shown in Figure 136 A-F will be found in pods about 3 mm. in length. 

 These may be put directly into the fixing agent, but later stages, 

 which are found in pods 5 mm. or more in length, should be trimmed, 

 as indicated in Figure 132 B, before fixing. Formalin-alcohol-acetic 

 acid is a good fixing agent; the Chicago chromo-acetic-osmic acid 



/0\ 



B 



D 



Fig. 136. — Capsclla bursa-pastoris: development of embryo. In D, dermatogen is shaded; in 

 E, both dermatogen and the plerome of the root are shaded, and the periblem of the root is com- 

 pleted; in F, dermatogen of root is completed. X520. From Chamberlain's Elements of Plant 

 Science (McGraw-Hill Book Co., New York). 



with 3 c.c. of acetic acid instead of 2 c.c, is also very good, and Dela- 

 field's haematoxylin stains better after the chromic series. Cut 5-10 n 

 thick and parallel to the flat face of the pod. 



For a study of the monocotyl embryo. Iris, and especially I. pseiida- 

 conis, can be recommended. The embryo is straight, and cotyledon, 

 stem-tip, and root are clearly differentiated before the endosperm be- 

 comes too hard to cut in paraffin. Fix pieces about 3 mm. wide cut 

 perpendicular to the face of the cheese-shaped seed. Do not try to cut 

 the whole pod. 



Sagittaria has been used quite extensively. It is easily obtained, 

 the whole head can be cut with ease, even after the cotyledon and 

 stem-tip are clearly differentiated, and the endosperm is instructive; 

 but, in later stages, the embryo is curved, like that of Capsella, so that 

 good views of the stem-tip are rare. 



