254 Methods in Plant Histology 



greenish cone within. At this time the sporogenous cells are easily 

 distinguished. Material collected in January, or at any time before 

 growth is resumed in the spring, shows about the same stage of 

 development. If it is desired to secure a series of stages with the 

 least possible delay, a branch bearing numerous clusters of cones 

 may be brought into the laboratory and placed in a jar of water. 

 Growth is more satisfactory in case of branches broken off in the 

 winter than in those brought in before there has been any period of 

 rest. The material can be examined from time to time, and a com- 

 plete series is easily secured. The mitotic figures in the pollen 

 mother cells furnish exceptionally instructive preparations. The 

 two mitoses take place during the last week in April and the first 

 week in May. Staminate cones which will yield mitotic figures can 

 be selected with considerable certainty by examining the fresh 

 material. Crush a microsporangium from the top of the cone and 

 one from the bottom, add a small drop of water and a cover to each, 

 and examine. If there are pollen tetrads at the bottom, but only 

 undivided spore mother cells at the top, it is very probable that 

 longitudinal sections of the cone will yield the figures. If a drop of 

 methyl green be allowed to run under the cover, it will enable one 

 to see whether figures are present or not. When desirable cones are 

 found slabs should be cut from two sides, in order that the fixing 

 agent may penetrate more rapidly and that infiltration with paraffin 

 may be more thorough. 



The later stages, showing the germination of the microspores, 

 furnish better sections if the cones are cut transversely into small 

 pieces about 5 mm. thick. It is very easy to get excellent mounts 

 of the pollen just at the time of shedding, which, in Pinus Laricio 

 in the vicinity of Chicago, occurs near the middle of June. Shake 

 a large number of cones over a piece of paper, thus securing an 

 abundance of material; then transfer to formalin alcohol. With 

 loose pollen grains, there is a great loss of material if any of the 

 chromo-acetic acid series, with the attendant washing, is used. If 

 the material is so abundant that plenty will be left after all the loss, 

 chromo-acetic acid may be used, and the mitotic figures, which may 

 still be found, are likely to stain more brilliantly than after formalin 



