Spermatophytes A ngiosperms 279 



An intermediate condition is seen in somewhat elongated embryo- 

 sacs of medium size, like those of Compositae. After a few free 

 nuclear divisions, walls appear simultaneously throughout the entire 

 sac. Silphium laciniatum is particularly good. Akenes from which 

 the corolla has just fallen will furnish material. 



The Embryo. The common Capsella bursa-pastoris (Shepherd's 

 Purse) is a favorable form for a study of the development of a dicotyl 

 embryo. The stages shown in Fig. 101, A-F, 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 Fig. 99, B, before fixing. 

 Formalin alcohol is a satisfactory fixing agent. Cut sections 5 to 

 10 IJL thick and parallel to the flat face of the pod. Delafield's haema- 

 toxylin, without any contrast stain, is excellent. 



For a study of the monocotyl embryo, Iris, and especially 

 I. pseudacorus, can be recommended. The embryo is straight and 

 cotyledon, stem-tip, and root are clearly differentiated before the 

 endosperm becomes 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 the embryo is curved, like that of Capsella, and good views are 

 rather rare. 



Zea Mays, especially the sweet corn, is a good type to illustrate 

 the peculiar embryo of the grasses. Directions have been given on 

 p. 264. 



In many forms good preparations of late stages may be secured 

 by soaking the seeds in water until the embryo bursts the seed coat. 

 Young seedlings furnish valuable material for a study of vascular 

 anatomy. 



