102 POLLEN. 



species, strings and filaments are woven from the anthers a centimetre long, and 

 insects visiting the flowers touch the strings, stick to them, and carry off with them 

 to another flower generally the whole of the contents of the anther in question. 

 The sticky substance is probably a mucilage formed from the outer wall of the 

 pollen-tetrad, or from the broken-down walls of the mother-cells. 



Not to be confused with the little pores communicating with the canals containing 

 the oil are the thin portions of the outer layer, into which the intine projects, caecum- 

 like, as it swells up in water. It often looks as though the extine were actually 

 perforated at these places; this, however, is not the case, and it is not till later, when 

 the intine pushes through and the pollen-tube is formed, that these places are burst 

 and true apertures arise. 



The variety exhibited by these spots is as remarkable as that of the sculpturings. 

 The outmost layer often thins out at those spots where the wall is grooved. As the 

 grain swells up in water, the extine often bursts at the thin region, and may 

 actually peel off (cf. fig. 217 10 ). In Mimulus and Thunbergia the thin region has 

 the form of a spiral, or it may run into loops and convolutions, as shown in 21 7 9 . 

 When the intine swells up and bursts the extine, the pollen-grain looks as though 

 it had been pared. In the Passion-Flo wer the thin places are ring-like, so that 

 with the swelling of the intine, the extine comes away in the form of little lids. 

 The same thing happens in the Gourd, where the lids are very small, and are 

 provided each with a little spine (fig. 217 3 ). A curious condition obtains in Morina 

 Persica (allied to the Teasel, cf. fig. 217 2 ). Each of the pollen-grains has at its 

 equator three projections, resembling closed bottle-necks with swollen, circular 

 mouths. Very frequently the thin places are disc-like, and may be compared to 

 the glazed port-holes of a ship. It is especially this form whicn suggests that the 

 outmost layer of the wall is perforated from the beginning. In Umbelliferae, 

 Rosaceae, Papilionaceae, Violets, Rutaceae, Hypericineae, Scrophulariaceae, and other 

 groups of plants, the little circular windows lie hidden in the grooves; in Cobcea 

 scandens (217 ^ they are in the "cells" of the honey -comb, and in Onagraceae, 

 e.g. Enchanter's Nightshade (Circcea), the outer coat is continued as a thin invest- 

 ment over the tops of the projecting warts (fig. 21 7 5 ). The number of windows 

 varies from plant to plant. Cyperaceae have one; Bromeliaceae and the Meadow 

 Saffron, Figs and Brugmansias two; Nettles, Oaks, and Beeches, Evening Primroses 

 and Willow-herbs, and many other plants three; Alders and Birches four to six; 

 Currants eight to twelve; Convolvuluses fifteen to eighteen; Carnations, Oraches, 

 and Mezereons twenty to thirty; and Nyctagineae over thirty. 



Having concluded the description of the walls of pollen-grains, the question 

 arises, for what purposes are all these remarkable structures, these grooves and 

 striae, these chinks and furrows, thorns and spiues developed? What is the meaning 

 of the coats of oil and viscin? What of the thin places, and windows, and tiny 

 lids? 



Of these the last question is the easiest to answer. As observation shows, 

 pollen-grains swell up with lightning rapidity when they are placed in water. The 



