380 VEGETABLli GROWTH. 



direction of the lines of fission. It is possible to distinguish 

 diiferences in the mode of division which are fairly charac- 

 teristic of Angiosperms and Gymnosperms, of Monocotyledons 

 and Dicotyledons. Although the morphology of the tissues 

 involved and the course of development are not yet complete!}- 

 understood, it may be said that the formation of pollen-grains 

 suggests throughout the mode in which the male elements are 

 produced in the higher cryptogams. 



(2) Changes in the Nucleus. The following suggestions by 

 Strasburger for demonstrating the nuclear changes in pollen- 

 grains can be applied with few modifications to all cases of cell- 

 division : Place the young part, in this case a yQvy young 

 anther, in a solution of methyl-green in acetic acid, and subject 

 it to slight pressure by which the contents of the anther-cells 

 will be discharged. Those parts susceptible of staining will take 

 the color readily and the different stages can be followed out sub- 

 stantiall}' as shown in the figures. For the staining-agent above 

 mentioned the following may be substituted, — gentian-violet in 

 acetic acid, or nigrosin with picric acid. Preparations made with 

 the latter can be preserved in glycerin without losing color. 



Another and better method is to place sections of the tissue 

 which has been kept for a few days in absolute alcohol, in an 

 alcoholic solution of safranin, and after twelve hours wash with 

 absolute alcohol ; then transfer them to oil of origanum and 

 thence to a thick solution of Damar in turpentine, for mounting. 



Tiy the safranin the delicate threads of the spindle are not 

 much colored ; they take, however, a good color with hsematoxy- 

 lin. Other combinations of coloring agents give good results.^ 



1001. Cell-division in plant-hairs. The stamen-hairs of Tra- 

 descantia Virginica afford excellent material for this examina- 

 tion. The last or upper three cells while still young are capable 

 of division. If the very j'oung hairs are transferred carefully to 

 a slide on which is a three per cent solution of cane-sugar, the}- 

 will continue the process of cell-division as shown in Fig. 167. 

 If the specimen is a good one, and has not been much injured 

 during its removal, it will remain active for several hours. 



All the examinations of cell-division require the use of high 

 powers of the microscope, none being better for the purpose 

 than the so-called homogeneous immersion lenses. 



1002. The direction in which the new cell- wall Is laid down at 

 the point of growth has been exhaustively examined b}- Sachs. 



1 Pas botanische Practieum, 1884, p. 598. 



