344 



WILD AND CULTIVATED COTTONS 



Homology 

 of the 

 grain. 



Structure 

 of the 

 grain. 



Many 

 tubes. 



Chief 

 types of 

 cotton 

 grains : 



tentative, and as given with the express object of inviting more 

 extended inquiry, especially on the part of investigators who may 

 have the opportunity of studying the pollen-grains of living plants. 



The pollen-grain, through its close homology, is often spoken of 

 as a spore (microspore), derived from a sporogenous mother-cell and 

 sporangium the anther. It is at first a nucleated spore that 

 becomes possessed in time of two portions a vegetative and a 

 generative. If placed on a drop of sweet fluid, the grain (spore) 

 germinates, and through its tube the protoplasm of the generative 

 cell, with one of its nuclei, passes into the egg-cell (oosphere) of the 

 ovule, thus causing fertilisation. This then is the modern conception 

 of the pollen-grain, and one that it is necessary to bear in mind, in 

 the study of these grains or spores, when prompted with a view to 

 discover methods of improvement of stock. 



The pollen-grains in Gossypium are, when fully grown, spheroid 

 in shape and, as with those of most other plants, may be spoken of 

 as possessing two coats or walls : an outer and thicker coat, the 

 exine (or, as it is sometimes written, the extine), and an inner thinner 

 and often transparent wall, the intine. These enclose, as the spore 

 contents, a thickened granular fluid. Under the influence of moisture 

 the inner coat would appear to swell more rapidly than the outer, 

 and at certain points thickens into hemispheric patches which 

 commence to press against the inner surface of the exine until they 

 ultimately penetrate through it. 



The species of Gossypium all agree in possessing numerous points 

 of emergence, hence in forming many tubes. According to some 

 writers pollen-grains have definite pores, specially contrived to 

 facilitate the passage of the germinating and elongating pollen tubes ; 

 by others these are merely thinner portions of the exine that become 

 dilated and finally ruptured by the vigorously growing tubes. As 

 seen under the microscope there would seem to be four main types of 

 cotton pollen-grains : 



(a.) Those in which the exine is greatly thickened and covered by 

 prominent warts, the bases of which are striated or buttressed, or 

 appear in certain attitudes like the teeth of a comb. On the summits 

 of these warts often arise variously shaped hyaline spines. Between 

 the spinose-warts are interspaces which seem closed in below by the 

 thin inner layer of the exine. The interspaces are, in these Gossypia 

 the pores or passages through which the pollen tubes emerge. This 

 condition is best seen in G, punctatum (see Plate 53, f. 16), in 



