392 XXVIII. THE ANDBCECIUM OF ANGIOSPERMS. 



seen. The formation of tubes takes place quite certainly and 

 rapidly in 5 per cent, solution of sugar and 1-5 per cent, gelatine 

 from the pollen-grains of Pceonia, Staphylea, and even from 

 Tradescantia when the pollen-grains are taken from freshly- 

 opened flowers. The most favourable objects are perhaps species 

 of Lathy rus, e.g., Sweet Pea, Everlasting Pea, etc., in 15 per 

 cent, solution of sugar and 1-5 per cent, gelatine. This solution 

 must be freshly prepared ; the sowing is best performed in a 

 suspended drop in a moist chamber (see p. 279). 



Though many pollen-grains have a wide range in this respect, 

 for most the degree of concentration of the sugar solution is of 

 vital importance, and ought therefore to be experimentally de- 

 termined for each species. A few further cases may be given. 

 Most species of Allium (onion) produce their pollen-tubes in a 

 3 per cent, solution of sugar, and show active protoplasmic 

 streaming. Tulipa Gesneriana requires from 1 to 3 per cent, 

 sugar ; pollen-grains of Leucojum astivum (snow-flake) germinate 

 very easily and rapidly in 3 to 5 per cent., and the same with 

 Narcissus poeticus (pheasant-eye narcissus) ; Convallaria majalis 

 (lily of the valley) in 5 to 20 per cent. ; Iris sibirica in from 30 to 

 40 per cent. The pollinia of orchids produce pollen-tubes in from 

 5 to 10 per cent, sugar, usually after from twenty to forty hours. 

 Amongst Dicotyledons the pollen-grains of Torenia asiatica germi- 

 nate very readily, producing pollen-tubes in 10 per cent, sugar 

 solution after about two hours, and the tube grows so rapidly that, 

 with a high power, its tip can be seen to move across the field 

 of view. Species of Gloxinia behave in much the same way in 10 

 per cent, solution ; Papaver (poppy), 5 per cent. ; species of 

 Sedum, and Viola tricolor (pansy), 30 per cent. In all these cases 

 1-5 per cent, of gelatine is added. The pollen-grains invert the 

 cane-sugar of the culture-(luid, the invertin being present in 

 the grain prior to germination. The pollen-grains likewise con- 

 tain diastase, and are capable therefore of dissolving starch-paste 

 if present in the culture-fluid, as, e.g., 1 per cent, potato starch- 

 paste, made at boiling temperature. This occurs whether they 

 have germinated or not, and even if they have burst in the culture- 

 fluid. 



Most pollen-grains burst if their cultivation is attempted in 

 water only. Some, however, produce their pollen-tubes in this 

 medium without bursting, e.g., those of Nicotiana (tobacco), Galeob- 

 dolon luteum ( = Lamium Galeobdolon, the yellow dead-nettle), 



