160 TYPE STUDIES 



Seed Plants. The outlines presented here will deal entirely 

 with the gametophyte generations and the organs of the sporo- 

 phyte, stamens and pistil (composed of carpels), especially con- 

 cerned with their development. For outlines covering general 

 flower structure see Sees. 44-46. An outline for a general type 

 study of an angiosperm such as the lily is presented in Sec. 162. 



141. The lily studied with reference to its gametophytes (App. 22). The 

 lily is a favorite subject for the study of pollen formation and the develop- 

 ment and fertilization of the embryo sac, partly for its clearness and partly 

 for the relative ease with which material may be obtained. Other types of 

 the lily family, such as the trillium, the tulip, the Roman hyacinth, etc., 

 are also good. Satisfactory studies on the gametophytes of the angiosperms 

 require microtome sections of the structures involved. Directions for the 

 preparation of these are outlined in Sec. 212. The wild lilies, such as 

 LUium philadelphicum, furnish excellent material, but various cultivated 

 lilies are equally good or better. 



A. The stamen of the lily. Dissect away the perianth of the lily flower to 

 show the stamens and pistil. 



1. Observe the arrangement of the stamens around the pistil. Draw 

 a stamen to show the stalk (filament) and the attachment of the 

 anther. 



2. Note how the pollen is discharged from the anther. 



3. Draw a pollen grain under h.p. to show the markings on its wall. 



4. Section the anthers and observe that the pollen is developed in four 

 locules, or pollen sacs, running lengthwise of the anther. 



5. In microtome sections properly stained (Sec. 212) note that the pollen 

 grains will show a large central nucleus, tube nucleus, and at one end 

 the generative nucleus, which gives rise later to two sperm nuclei. 



■B. The development of pollen. To obtain the stages of pollen formation 

 anthers must be taken from very young unopened buds, the stamens 

 of which when cut across exude a watery fluid from the pollen sacs. 

 Should the fluid be milky or yellowish the stamen is too old and will 

 certainly contain pollen grains. Stamens of the proper age must he 

 fixed, imbedded, and cut lengthwise on the microtome and stained as 

 described in Sec. 199, D. Such preparations will show various stages in 

 development and division of the pollen mother cells to form the pollen 

 grains in groups of four, or tetrads. The following conspicuous stages 

 are likely to be found and should be drawn. 



1. The spore mother cells before division, with their nuclei in a resting 

 condition. They constitute a spore-forming tissue (archesporium). 



