250 Methods in Plant Histology 



After the embryos begin to grow down into the endosperm, 

 oblong pieces containing the embryos should be cut out. 



After the cotyledons appear, useful preparations may be made by 

 dissecting out the entire embryos, which may be fixed in chromo- 

 acetic acid, washed, stained in eosin or in Delafield's haematoxylin, 

 placed in 10 per cent glycerin, and mounted by the Venetian turpen- 

 tine method. Since the suspensors become long and irregular, each 

 embryo should be placed in a separate dish, lest the suspensors 

 become entangled and broken. 



After the stony layer becomes hard, it is better to use a small 

 fret saw for opening the ovule. Before the embryo has pushed down 

 into the endosperm, the ovule should be sawed in two transversely. 

 The endosperm and nucellus can then be picked out and treated as 

 desired. After the tip of the embryo reaches the middle of the endo- 

 sperm, the ovule should be sawed open longitudinally. 



GYMNOSPERMS GINKGOALES 



From the standpoint of technic, the Ginkgoales, now represented 

 only by Ginkgo biloba, are less difficult than the Cycadales, but the 

 difficulties are somewhat similar. 



The Vegetative Structures. -The adult stem is harder to cut than 

 Pinus, but good sections should be secured by boiling in water and 

 soaking for a few clays in glycerin. Transverse sections of the "spur" 

 shoots are easily cut. They have a comparatively large pith and 

 narrow zone of wood, thus contrasting sharply with a long shoot of 

 the same diameter, which has a small pith and wide zone of wood. 



Spermatogenesis.--The entire staminate cone, even at the time of 

 shedding pollen, can be cut in paraffin. For the latest stages, how- 

 ever, it is better to remove the sporophylls and cut them separately, 

 since the sections must not be thicker than 5 p, if they are to show 

 the internal structures of the pollen grain. 



The young staminate cones become recognizable in June; by 

 September, they have nearly or quite reached the spore mother-cell 

 stage, but the division of the spore mother cell does not take place un- 

 til the following April. In these early stages the bud scales should be 

 carefully dissected away before fixing. Pollen is shed early in May. 



