120 PRACTICAL BOTANY 



The body of the inverted ovule consists of 



b. Two integuments, each several layers of cells in thickness, 

 the outer being united with the funiculus : the integuments 

 cover the body of the ovule completely, excepting a narrow 

 channel (micropyle) near its apex. Within the integuments 

 lies 



c. The nucellus, an oval mass of cellular tissue in which is 

 embedded 



d. The embryo-sac (macrospore), a large oval cell, situated 

 centrally a short distance below the apex of the nucellus. 



Examine the embryo-sac with a high power, and observe 



1. The granular, vacuolated protoplasm which fills it : 

 embedded in this are to be found 



2. A large central nucleus, with highly refractive nucleolus. 



3. At the micropylar end of the embryo-sac, three cells, with 

 clearly defined nuclei. Two of these (the synergidse) fill the 

 apex of the sac, the third (the ovum) being placed laterally, 

 a little below the apex. 



4. At the posterior end of the sac are three cells (the antipodal 

 cells), also with clearly defined nuclei. Divisions of these cells 

 occasionally occur, so that their number may be fonnd to be 

 greater than three. 



Note the tapetum, consisting of cells more or less disorganized 

 which partially or completely surround the embryo-sac. 



FERTILIZATION. 



I. Remove from flowers of Stellaria media, which have just 

 faded, the three styles : moisten them with alcohol, and mount 

 quickly in water : note the cylindrical colourless styles, curved 

 at their upper ends : the stigmatic surface with its numerous 

 papillose hairs is found on the convex side of the curved part of 

 the style. Note especially the numerous yellow pollen-grains 

 adherent to the stigmatic surface, while it may often be seen 

 that a pollen-tube will proceed from the pollen-grain, and 

 enter the tissue of the style. 



II. The style and stigma of flowers of Rhododendron ponticum 

 from which the corolla has already fallen off will also be found 



