368 



METHODS IN PLANT HISTOLOGY 



^.-Endosperm 



-Embryo 



In rather long, narrow embryo sacs, a cell wall is likely to follow 

 even the first division of the endosperm nucleus, so that the endosperm 

 is cellular from the beginning. Ceratophyllum, Monotropa, and Ver- 

 bena will furnish material 

 of this type. 



In large, broad embryo 

 sacs, the formation of en- 

 dosperm is almost sure to 

 be initiated by a series of 

 simultaneous free nuclear 

 divisions. This is a difficult 

 condition to fix well, since 

 the layer of protoplasm, 

 with its free nuclei, sur- 

 rounding a big vacuole, is 

 likely to shrink away from 

 the surrounding cells. As 

 the free nuclear period 

 comes to a close, walls 

 appear at the periphery, 

 and wall formation gradu- 

 ally advances toward the 

 center until the entire sac 

 isfilled with tissue. Lilium, 

 Capsella, and Ranunculus 

 furnish examples of this 

 type (Fig. 135). 



An intermediate condi- 

 tion 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. — In most angiosperms the endosperm divides earher 



than the fertiUzed egg and in some cases, Uke Asclepias and Casuari- 



na, the free nuclear stage of the endosperm is completed and the cellu- 



—Micropyte 

 Stalk (Funiculus) 



Fig. 135. — Capsella bursa-pastoris: ovule (megasporan- 

 gium) with embryo and endosperm, the embryo wath 

 plerome, periblem, and dermatogen differentiated. Cell 

 walls are appearing in the endosperm. X60. From Cham- 

 berlain's Elements of Plant Science (McGraw-Hill Book 

 Co., New York). 



