130 THE PLANT CELL. 



Note. — The study of the development of the microspore may be readily 

 carried out in sections of the young flower of Polygonatum. Longi- 

 tudinal sections of the fixed and hardened flower buds will out across the 

 young anthers. Sections should be stained with safranin and hsematoxylin 

 to show up the mitotic figures in the divisions of the pollen mother-cells. 

 The study of the maturation of the microspore is difficult to carry out, and 

 rather beyond the scope of the practical work noted in these pages. 



The development of the embryo-sac and its maturation stages can, 

 however, be readily studied in Lilium martagon, using young flower-buds. 

 Jlellehorus niger is also a useful plant for the later stages, as also is Caitha 

 paliistris. Transverse sections of the young ovary of Lilium will cut 

 across the ovules and erabryo-sac longitudinally, and often in the same 

 section three dififerent stages may be recognised. 



The development of the embryo is best observed in Capaella bursa- 

 pastoris. In this plant the early growth of the embryo may be made out 

 by selecting ovaries of various sizes, placing them in glycerine and water, 

 and gently squeezing them, in order to flatten the ovules and force out 

 the embryos. Development of endosperm can be studied by taking trans- 

 verse sections of the ripening carpels of Oaltha (fixed and hardened), and 

 staining to show up the nuclei, lying free in the cytoplasm of the embryo- 

 sac. Helleboncs niger may also be used for this purpose. 



Fig. 99. — Structure of the Immature Micbospore. — w, Extine ; 

 i, intine ; n, nucleus lying in the cytoplasm ; lb, lateral lobes formed 

 from the extine. 



B. Reproduction in Gymnosperms. 



The Gymnosperms are interesting from the fact that in them 

 the reproductive cycle forms a sort of link between the process as 

 it occurs in Angiosperms and that taking place in the hetero- 

 sporous Pteridophyta. The Cycadeae show perhaps the most 

 resemblance in this respect, but the type here selected will be 

 Pinus, in which genus all the more important details may be 

 readily made out. 



As in the Angiosperms, the microspore (pollen-grain) and the 

 macrospore (embryo-sac) form the primary sexual cells, and in 

 each of these certain processes of maturation take place, which 

 lead to the formation of the effective cells in reproduction. 



