Spermatophytes Gymnosperms 255 



alcohol. However, most of the mitoses take place before the pollen 

 is shed or after it reaches the nucellus. Infiltration in the bath will 

 not require more than 30 minutes. When the infiltration is com- 

 plete, there should be only enough paraffin to cover the mass of pollen 

 grains. The material may now be poured out into a rectangular dish 

 or box with surface enough to make the cake about f inch thick. 

 Good results may be secured by pouring the paraffin upon a cold 

 piece of glass. Another method is to keep the material in a small 

 bottle during infiltration, and when ready to imbed, simply cool the 

 bottle. Break the bottle carefully, cut off the lower portion of the 

 paraffin containing the pollen, mount it on a block in the usual 

 manner, and trim away some of the paraffin so that two parallel 

 surfaces will make the sections ribbon well. Sections should not be 

 thicker than 5 IJL. Material in this stage shows a large tube nucleus, 

 a somewhat lenticular (generative) cell with a more deeply staining 

 nucleus, and, lastly, two small prothallial cells quite close to the spore 

 wall. The prothallial cells cannot always be detected at this stage, 

 and there may be some doubt as to whether two such cells are always 

 present. The division of the lenticular cell into "stalk ceH' : and 

 "body cell," and also the division of the body cell into the two male 

 cells, must be looked for in sections of the nucellus of the ovule. 



Abies balsamea is a better type for illustrating spermatogenesis, 

 since the pollen mother cells and the pollen grains are much larger 

 and the division of the generative cell into the "stalk" and "body' 

 cells takes place before the pollen is shed (Fig. 88). 



Araucaria and Agathis are the best forms for illustrating numerous 

 prothallial cells. Podocarpus and Taxodium are also good. Thuja or 

 Juniperus may be used to illustrate the entire absence of prothallial 

 cells. Both Thuja and Juniperus show highly organized male cells. 



Oogenesis. In Pinus Laricio the rudiment of the ovulate strobi- 

 lus, 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. 



