332 



METHODS IN PLANT HISTOLOGY 



The young staminate cones become recognizable in June; by Sep- 

 tember, they have nearly or entirely reached the spore mother-cell 

 stage, but the division of the spore mother-cell does not take place 

 until the following April. In these early stages the bud scales should 

 be carefully dissected away before fixing. Pollen is shed early in May. 

 Fix in chromo-acetic acid, with or without a little osmic acid, cut 5 n 

 thick, and stain in iron-alum haematoxylin. There are four cells in the 

 pollen grain at the time of shedding (Fig. 115). 



For the development of the micro- 

 sporangium, consult Dr. Starr's paper. 

 Pollen tubes and their structures 

 must be studied in sections of the nu- 

 cellus. Fertilization, in the Chicago 

 region, occurs about the middle of 

 September. 



Oogenesis. — Young ovules about 

 0.25 mm. in length are found about 

 the middle of April; the megaspore 

 mother-cell and its division into four 

 megaspores are found about the first of 

 May; the free nuclear stage in the de- 

 velopment of the female gametophyte 

 extends from the first week in May to 

 the first week in July; during July, 

 walls appear; then come the archego- 

 nium initials and the growth of the 

 archegonium, the ventral canal cell 

 being cut off the second week in September; fertilization, free nuclear 

 division in the sporophyte, and the beginning of walls may all be found 

 before the end of September; cotyledon stages belong to October, 

 and when the seeds fall in November the embryo extends throughout 

 nearly the entire length of the endosperm. This is the winter resting 

 stage, but, planted in the greenhouse, the seeds germinate without 

 any resting period, as in the case of cycads. 



For all stages in oogenesis and development of the embryo, use the 

 Chicago chromo-acetic-osmic solution. If the gametophyte in early 

 free nuclear stages should shrink, increase the acetic acid up to 4 c.c. 

 and decrease the chromic acid to 0.7 or even 0.5 c.c. If the chromic acid 

 is decreased, lengthen the time to 48 hours. Free nuclear stages, like 



Fig. 115. — Microspores of Gingko 

 biloba: A, first prothallial cell. B, first 

 and second prothallial cells. C, the two 

 prothallial cells and the mitosis which will 

 give rise to the body cell and tube cell. 

 D, the two prothallial cells, the lower 

 nearly disintegrated, the body cell, and 

 the tube cell. Pollen is shed in the condi- 

 tion shown in D. X770. 



