474 Transactions of the Society. 



divide and give rise to the pollen-mother-cells, which fill the pollen- 

 sacs. The inner layers of the anther- wall, as a rule, dissolve, thus 

 forming a fluid mucilage. 



By degrees the walls of the pollen-mother-cells thicken, and 

 the protoplasm divides into four parts, each of which becomes a 

 pollen-grain. These perhaps correspond with the egg and the three 

 polar bodies, but whereas the three polar bodies appear to take no 

 further part in the life-history, the four pollen-grains are all alike. 

 In some cases they remain attached, thus forming " tetrads." 



When the pollen has reached the stigma it begins to germinate. 

 The nucleus divides, and the intine emits a tube, as was first 

 observed by Gleichen, which gradually finds its way to the ovule. 

 Amici suggested in 1830 that the tube grows by absorbing nourish- 

 ment from the pistil; in fact like the hyphse of a fungus, and this 

 was confirmed in the following year by E. Brown. The two nuclei 

 both enter the tube, and one coalesces with the corresponding female 

 nucleus of the ovule, thus fertilizing it. The other nucleus is said 

 to be without function and to die away ; but why, then, should it 

 have been produced ? 



EXPLANATION OF PLATES VII. and \U1.— continued. 



Fig. 11. — TroxJteolum majus Linn, x 400. Side view of pollen, showing one 



furrow on the highest point or ridge. 

 ,, 12. — Ditto. End view of pollen, showing the three furrows on the angles. 

 ,, 13. — Viola lutea Huds. x 400. Long axis of the pollen. FP, two furrows 



appearing close to the margin, but actually cutting it away. 

 ,, 14. — Ditto. End view of the pollen. FFFF, the four furrows, two of which 



are discernible on anj' one of the four faces. 

 „ 15. — Leo7itodon hispidum Linn. I. Pollen-grain, x 800. 

 ,, 16. — LeontodonhisjyidumLiinn. x 25. A, top of the anther-cylinder ; St, style ; 



AS, AS, arms of the st3'le ; SS, SS, the stigmatic surfaces. The pollen 



on the style and its arms is x 50. 

 „ 17. — After Purkinge (only for size). Pollen of Elymus sabulosus. 

 „ 18. — ,, Bromus erectus. 



,, 19. — ,, Avena latifolia. 



,, 20. — Ranuncuhis aciis Linn, x 400. Pollen-grain newly liberated from the 



anther. 

 ,, 21. — Ditto. Side view of pollen after desiccation. 

 ,, 22. — Ditto. End view of pollen, showing three rounded lobules or ridges, 



with the three furrows between. 

 ,, 23. — Aconitum Napellus Linn, x 400. Side view of pollen, showing one 



median furrow. 

 ,, 24. — Ditto. End view, showing the furrows meeting on the top. 

 ,, 25. — Ranunculus sceleratus Linn, x 400. Spherical pollen, as it first leaves 



the anther. 

 „ 26. — Ditto. End view of a larger pollen. 

 ,, 27. — Ditto. End view of pollen, showing the three rounded lobules or ridges 



of the grain, with the three furrows between. 

 ,, 28. — Aquilegia chrysantha A. Gray, x 400. Side view of pollen, showing 



one furrow. 

 ,, 29. — Ditto. End view, showing all three furrows. 

 ,, 80. — Ranunculus acris Linn. x 400. Spherical permanent form, very 



numerous and very variable in dry weather. 

 ,, 31. — Ditto. Ellipsoid form, scarce under similar conditions — side view. 



