PROTECTION OF POLLEN. 



every degree of cohesiveness in pollen dusty, floury, crumbly, clotted, glutinous, 

 waxy. A marked contrast is noticeable between flowers the anthers of which 

 produce dusty, and those which produce coherent pollen. So pronounced is this, 

 that we shall treat of the pollination of these flowers, and in particular of the 

 transmission of the pollen from flower to flower, under separate headings. Here it 

 need only be added that this distinction between dusty and coherent pollen is 

 found not only with isolated pollen-grains but with tetrads. When the stamens 

 of Heaths (Erica) are disturbed the pollen escapes in clouds of dust, just as it does 

 from the catkins of the Hazel. This dust, however, consists, not of isolated poHen- 

 cells, but of tetrads. In Azaleas and Rhododendrons, on the other hand, the 

 pollen-tetrads cling together into sticky filaments, just as do the isolated grains 

 of the Evening Primrose and Willow-herb. 



Why it is that the pollen is in some cases in tetrads and in others in isolated 

 grains, why its adhesiveness is promoted by such various means as those enumerated, 

 is difficult to say. These differences are perhaps connected with the varying form 

 of the insect-visitors which carry the pollen away, and of the stigmas upon which 

 it is deposited. That the sculp turings protect the pollen against untimely wetting- 

 will be shown in the following chapter. 



PROTECTION OF POLLEN. 



The approach to Venice from the mainland is by a long embankment, on either 

 side of which the traveller commands an endless vista of marshes full of reeds and 

 rushes broken here and there by expanses of brackish water the famous lagoons 

 which themselves exhibit a luxuriant vegetation consisting principally of Pond- 

 weeds and Naiadaceae. One plant in particular, the Grass-wrack (Zostera), is 

 conspicuous for its abundance in the lagoons, covering, as it does, extensive tracts 

 of the sandy mud at the bottom of the shallow water. The leaves are submerged, 

 ribbon-shaped, and of a brownish-green colour somewhat resembling sea-weed, and, 

 when collected and dried, they are known in commerce by the name of " Sea-grass", 

 and are used in the packing of glass, and of late years also for stuffing mattresses 

 and cushions. These Grass-wracks, of which there are two known species, differ 

 so greatly from other Phanerogams, not only in appearance, but also in development 

 and in the mode of pollination, that one might almost be induced to assign to them 

 and their immediate allies a special class, were it not that the fact of the existence 

 of numerous intermediate forms and connecting links tells against their isolation. 



In the first place, the pollen in Zostera does not possess the outer coat which is 

 so characteristic of the cell-membranes of most pollen-cells. Further, from the 

 moment the pollen-cells are set free by the opening of the anthers an event which 

 occurs under water they exhibit the form of elongated cylindrical tubes. In the 

 plants most nearly related to the Grass- wracks, namely, the genera Posidonia and 

 Cymodocea, some species of which grow in brackish and some in salt water, the long 

 hypha-like pollen-cells lie in complicated coils and curves within the anther, and 



