DEVELOPMENT OF THE POLLEN-GRAIN. 



385 



of the sac, takes on an intense yellow-brown colour, a glistening 

 oily appearance, and so forms the oily substance which clings 

 around and upon the pollen-grains. 



Substitutes for Hemerocallis. The species of the genus 

 Lilium agree with Hemerocallis. The processes of differentia- 

 tion in the anther take place here, however, later. In flower- 

 buds of the white Lily, Lilium candidum, of L. croceum and 

 others, four-fifths of an inch high, the pollen mother-cells first 

 begin to divide. In cross-sections through fresh flower-buds 

 the large tapetal cells are very striking from the yellow-brown 

 colour of their contents. The hypodermal cells, as well as all 

 the others which are later on provided with thickening ridges, are 

 densely filled with starch grains. Funkia ovata (May to July) 

 provides likewise a very favourable object for study, and agrees 

 with Hemerocallis and Lilium, as also do Agapanthus umbel- 

 latus (Greenhouse, April), species of Iris, and many others. 

 Tulipa (the Tulips, April, May), and Hyacinthus orieutalis 

 (the Hyacinth, January to May) are likewise good to use. In 

 Tulipa the filament under the anther tapers so sharply that 

 this latter will draw off; in Hyacinthus the anthers are almost 

 sessile on the perianth segments. 



Pollen-Grains of Tradescantia. Tradescantia virginica (obtain- 

 able from early summer to frost) does not cut so well, but we 

 examine the flowers for their pollen-grains. The stamens of a 

 bud which is ready to expand show us the beautiful sulphur- 

 yellow anthers, fixed upon 

 violet filaments covered 

 with violet hairs (in which 

 we have already studied 

 protoplasmic circulation). 

 The dry pollen-grains are 

 folded (or grooved) on one 

 side (Fig. 141, A). In 

 water the fold is levelled 

 out, and the grains become 

 almost ellipsoid ; but the 

 side which was furrowed 

 is more strongly bulged. 

 Its exine is decorated with fine sinuous lines ; the furrowed 

 side also shows this structure, but the exine is there more feebly 

 developed. In the finely granular contents can be distinguished 



25 



FIG. 141. Tradescantia virginica. ^.pollen- 

 grain dry. B, in water. 0, young pollen-grain 

 in water, showing the reproductive and vegetative 

 cells (x 540). 



