154 WAKE-ROBIN, 



g. Make drawings illustrating the above stages. 

 5. The oospore and embryo sporophyte. The microspores 

 are carried to the stigma, and from them pollen-tubes grow 

 down through the style and ovary, and into the micropyle 

 of the ovule. As the tube pushes its way through the 

 nucellus into the micropylar end of the embryo-sac, 1 the 

 male gametes migrate to the end. Arrived here the tip 

 of the tube opens, the male gametes pass out and one of 

 them unites with the egg, thus producing the oospore. 2 

 By means of sections similar to those used in 4, except that 

 they are made from slightly older ovules, observe: 



1. The oospore. Note whether the synergids are present, 

 and whether the primary endosperm nucleus has begun 

 to divide to form the endosperm. 



2. A stage in which the oospore has divided to form the 

 embryo. Note the suspensor cell that attaches the em- 

 bryo to the micropylar end of the sac. 



3. A stage in which the suspensor has become considerably 

 elongated, thus pushing the embryo proper well into the 

 embryo-sac. Note the condition of the endosperm calls 

 at this time. 



4. In a practically mature seed observe the much larger 

 embryo about which the endosperm cells are closely 

 packed. 



5. Draw, illustrating the above stages. 



By use of Gray's "Manual of Botany" or Britton's "Flora 



1 It has been shown by different investigators that in several Angio- 

 sperms of rather low rank the pollen-tubes may pass through the basal 

 (chalazal) end of the ovule and then traverse the length of the ovule 

 to its nucellar end, thence into the embryo-sac in order to effect fer- 

 tilization. Such a condition is known as chalazogamy. 



2 The research work of the past few years indicates that double fer- 

 tilization is not uncommon and is probably the rule in Angiosperms. 

 It has been shown in many plants that one of the male cells unites 

 with the egg, while the other passes down to the primary endosperm 

 nucleus and unites with it. 



