152 WAKE-ROBIN. 



b. That the four sporangia are ready to open in pairs, 

 thus forming two pollen-sacs. 



c. The thinness of the sporangium wall, and the place 

 especially formed for opening the anther. The act of 

 opening the sporangia is known as dehiscence. 



d. The microspores (pollen-grains). 



e. Diagram the section. 



2. The formation of male gametes. In many cases the micro- 

 spores will have germinated before they have left the an- 

 ther. In germination the spore nucleus divides, thus form- 

 ing a so-called generative nucleus and a tube nucleus. The 

 latter is associated with the development of a pollen-tube. 

 The generative nucleus finally divides, thus forming two 

 male gametes. It is the rule for this last division not to 

 occur until after the pollen-grain has left the anther. The 

 results of the first division may often be observed within 

 the walls of microspores still in the anther. 



3. The carpels, megasporangia (ovules), and megaspores. 

 By means of cross-section of the ovary 1 observe: 



a. Ovules that appear as somewhat oval bodies attached 

 by their bases to the central axis of the ovary. In 

 Trillium the ovules are curved, having their tips turned 

 toward their bases and also toward the central axis of 

 the ovary. Selecting one good ovule, observe: 

 i. The outer covering of the ovule, the integuments. 

 ii. The opening between the ends of the integuments, 



the micropyle. 

 iii. The tissue of the ovule beneath the integuments, 



the nucellus. 

 iv. Within the nucellus the single large megaspore, or 



the embryo-sac that has developed from it. 

 v. Diagram the section. 



1 A good set of permanent serial sections of the ovules illustrating 

 developmental stages should be at hand. 



