SPERMATOPHYTES— GYMNOSPERMS 34 1 



or Juniperus may be used to illustrate the entire absence of prothallial 

 cells, a very advanced condition; while both these genera have highly 

 developed sperms, like those of the cycads and Ginkgo, except that 

 they lack cilia. This is a good illustration of the fact that one structure 

 may advance while another remains primitive. 



Oogenesis. — In Pinus laricio the rudiment of the ovulate strobilus, 

 which is to be pollinated in June, can be detected in the preceding 

 October. The collection of this stage is very uncertain, because there 

 seems to be no mark distinguishing buds containing ovules from buds 

 which are only vegetative. By collecting numerous buds from the tops 

 of vigorous trees which are known to produce an abundance of strobili, 

 a few buds containing the desired stages may be obtained. In May, 

 after the strobili break through the bud scales, material is easily col- 

 lected. Up to the time of pollination the entire ovulate strobilus cuts 

 easily in paraffin. Longitudinal sections of the cone at this time give 

 good views of the bract and ovuliferous scale bearing the ovules. The 

 integument is very well marked, and in the nucellus one or more 

 sporogenous cells can usually be distinguished. The young cones of 

 Pinus contorta are very interesting since the megasporangium often 

 contains several megaspore mother-cells, most of which may divide, so 

 that the structure really looks like a sporangivnn. 



As soon as the scales close up after pollination, the cone begins to 

 harden and soon makes trouble in cutting. Even before the scales close 

 up, it is better to cut a slab from opposite sides of the cone; after the 

 scales close, it is almost a necessity. For sections of the whole cone, fix 

 in formalin acetic alcohol. Dr. Hannah Aase succeeded in cutting 

 complete series of paraffin sections from cones of Pinus hanksiana 

 more than 2 cm. in length. 



She fixed them in formalin alcohol, and used prolonged periods in 

 dehydrating, clearing, and infiltrating. Land's dichromate of potash 

 and glue fixative was used in fixing the sections to the slide. Such 

 series of sections of large cones were necessary for an investigation of 

 the vascular anatomy. 



For a study of the ovule and the structures within it, cut the pair of 

 ovules off from the scale with extreme care, because the young mega- 

 gametophyte is so difficult to prepare without shrinking that it would 

 be a pity to have any imperfections due to careless pressure. These 

 difficult free nuclear stages begin in the autumn, are interrupted by 

 winter, and are completed in May. 



