96 



POLLEN. 



filled with the archesporial cells — is easily recognized. As time goes on the 

 contrast between the wall of the anther and the contents of the chambers becomes 

 more pronounced. The archesporial cells divide, giving rise to the pollen-mother- 

 cells which entirely fill the pollen-sacs. Of the layers of the anther-wall, the 

 inmost is usually dissolved, so that the mother-cells are bathed in a fluid mucilage; 

 thus the wall comes to consist solely of the outmost, enveloping layer and of the 

 contractile cells ("fibrous layer") within. 



Changes continue in the chambers or pollen-sacs, and in the partition-walls 

 between them. The walls of the pollen-mother-cells become thickened, and often 

 show a stratification. The protoplasm within divides into four parts, arranged 

 frequently, though not invariably, in the corners of a 4-sided pyramid (i.e. in 

 tetrads). Each of these cells becomes invested with a wall of its own, at first thin 

 and delicate, but afterwards thickened and stratified. These are the pollen-grains. 

 Their protoplasm possesses the property of a fertilizing agent, and is termed the 

 Sperm.ato'plas'in. 



In most plants a further division of the protoplasm in the pollen-cells takes 

 place. This is conspicuous in the Conifers and Cycads, but relatively obscure in 

 the majority of flowering plants. Of the two or more cells thus arising within the 

 pollen-grain one only takes an actual part in fertilization. 



How long the spermatoplasm retains its fertilizing properties unimpaired has 

 not been sufficiently investigated. It has been stated of the plants enumerated 

 below that this property is lost as follows: — 



In Hibiscus Trionum ... ... after 3 days. 



„ The Wallflower {Gheiranthus Cheiri) „ 14 „ 

 „ „ 'Baxisy {Viola tricolor) ... „ 26 „ 

 „ „ 'Bw^e {Ajuga reptans) ... „ 32 „ 



In the Larger Periwinkle ( Vinca 



major) after 43 days. 



„ PoBonia puheiis „ 58 „ 



„ Pceonia tenuifolia ... „ 65 „ 



„ Clivia nobilis ,, 76 „ 



It is by no means an unusual thing for gardeners to send the poUen of Cycads 

 and Palms for fertilization to distant countries without its properties being 

 impaired, provided it is kept dry during transit. The Arabs, who artificially 

 pollinate the female flowers of the Date-palm, put aside some of the pollen from 

 year to year, so that, in the possible event of the male flowers not developing, they 

 may ensure a crop of dates. According to tradition, the pollen of Date-palms, 

 Hemp, and Maize, can be used effectively for artificial pollination even after a lapse 

 of eighteen years. Unfortunately, reliable investigations are wanting to show 

 whether these accounts belong to the realm of gardeners' stories or not. 



A great diversity obtains as to the manner of coherence of the pollen. When 

 the walls of the mother-cells, containing the pollen-grains, become entirely absorbed, 

 the pollen-sacs are filled with isolated cells, a condition which may be described as 

 free pollen. Even now numbers of adjacent pollen-grains may cohere in clusters, 

 in consequence of their possessing sticky coats or other arrangements. But in this 

 case there is no suggestion of its being a real tissue, a diflTerence of some moment. 



In many plants, on the other hand, the pollen-cells remain, as they arose in the 



