362 



METHODS IN PLANT HISTOLOGY 



Oogenesis. — As in spermatogenesis, the early stages will be found 

 in preparations of floral development. The preparations of Capsella 

 will show the origin and development of the nucelliis (megasporan- 

 gium) and also the megaspore mother-cell. The division of the mega- 

 spore mother-cell to form four megaspores takes place shortly before 

 the bud begins to unfold. A massive megasporangium with several 



; , y •/ 



"^f.^'^A# 



■^-^n"-^"* ' fT-"¥ I^W *^'? '4^1^ .i-L'.. 



:^^-^ *^' 



.*S 



v^t' 













/^ 



-< 



Fig. 129. — Lilium canadense: photomicrograph of part of transverse section of ovary, showing 

 longitudinal section of ovule with megaspore mother-cell, just before the formation of integuments. 

 Fixed in chromo-acetio acid and stained in iron-alum haematoxylin. Eastman Commercial Ortho 

 film, Wratten H filter (blue); Spencer 4-mm. objective, N.A. 0.66; ocular X6; arc light; exposure, 

 2 seconds. Negative by Dr. P. J. Sedgwick. X400. 



megaspore mother-cells may be found in Ranunculus; a megasporan- 

 gium with only one megaspore mother-cell and only one layer of cells 

 surrounding it may be found in any of the Compositae. Erigeron 

 philadelphicus is very good. Head from 4-6 mm. in diameter will show 

 the stages from the megaspore mother-cell to the four megaspores. 

 E. strigosus and E. annuus are equally good. In Trillium and in Cypri- 

 pedium the embryo sac is formed from two megaspores, which are not 



