84 SUMMARY OF CURRENT RESEARCHES RELATING TO 



BOTANY. 



A. GENERAL, including Embryology and Histology of the 

 Phanerogamia. 



Development of the Pollen of Orcliidese.* — L. Guignard has 

 investigated the structure and mode of development of the pollen in 

 Orchideae, especially in the Ophrjdeai and Neottie^. The jn-imordial 

 mother-cells of the pollen are formed from a hypodermal layer, each 

 of whose cells divides into an outer and an iuner cell, the latter 

 becoming the pollen mother-cells. They constitute, therefore, at first 

 a simple layer, and are distinguished from the other cells by their 

 denser cell-contents. Each of these cells further divides in different 

 directions, and developes into one of the " massulte," the peripheral 

 cell-walls of which are much thicker than the inner walls. The outer 

 hypodermal layer also undergoes numerous divisions, dividing into an 

 inner layer, the " tapete," and an outer layer the walls of which are 

 strongly thickened, but do not, as in most other plants, become fibrous. 



The further development of the mother-cells of the pollen more 

 closely resembles in some respects that of dicotyledons than of other 

 monocotyledons. The cells do not divide completely, but only their 

 nuclei, not even a cell-plate being formed. The two nuclei again 

 divide into four, which are arranged either in one plane or in a 

 tetrahedron. The true membrane of the pollen-grains is formed 

 nearly simultaneous, on the side towards the mother-cell-wall, and in 

 the equatorial plane of the nuclear spindle. It often has a granular 

 extine composed of two layers, the outer one of which, however, 

 occurs only at the periphery of the tetmhedron. Dui-ing its formation 

 the mother-cell-wall becomes absorbed. 



The author confirms Strasburger's andElfving's account of the part 

 played by the two nuclei in the act of impregnation. The process 

 can be very well followed out with the assistance of colouring reagents, 

 either in the process of fertilization itself, or by making the pollen- 

 grains germinate in a 2 per cent, solution of sugar. The tetrahedra 

 of Neottia ovata and nidus-avis are specially favourable for observa- 

 tion. Colouring with hasmatoxylin immediately after the pollen- 

 tube has burst through the extine shows the larger nucleus occujDyiug 

 the swollen extremity ; at some distance is the elongated granular 

 vegetative nucleus. At the moment of impregnation this latter has 

 almost entirely disajipeared, while the larger nucleus becomes resolved 

 into an amorphous substance which is still coloured by hasmatoxylin, 

 and passes through the thin wall at the extremity of the pollen-tube. 

 Under artificial conditions the extremity of the tube may often be 

 seen to be perforated. The protoplasm of the pollen-grain has in the 

 meantime entered the tube along with the nuclei, and becomes aggre- 

 gated into balls. 



Formation of the Pollen-grains in Gymnosperms.t — L. Jui-anyi 

 has investigated the details of the mode of formation of the pollen in 



* Ann. Sci. Nat. (Bot.), xiv. (1882) pp. 26-45 (1 pi.). 

 t Bot. Ztg., xl. (1882) pp, 814-8, 835-44. 



