POLLEN-GKAINS OF GTMNOSPEEMIA. 625 



(p. 549) of the cell-contents may be observed, which is likewise occasion- 

 ally to be seen in recently formed pollen-tubes of other plants. The 

 minute starch-grains of the cell-contents are noticeable as exhibiting a 

 molecular motion, which was at one time imagined to be of vital cha- 

 racter, and might lead the inexperienced to suspect the existence of 

 minute spermatozoids. 



Examination of Pollen. Pollen-grains should be examined first as 

 dry or opaque objects, as their form and dimensions are altered by en- 

 dosmosis when immersed in fluids. Oil of cloves, syrup, glycerine, or 

 naphtha are convenient fluids for examining pollen. A large number of 

 pollen-cells, illustrative of their form and size, are given in the ' Gardeners 

 Chronicle,' 1876, pp. 516 and 548. The discrepancies in the descriptions 

 given by various authors depend on the conditions under which the pollen 

 is examined. In the anther, and immediately after expansion, it is gene- 

 rally globular, but it often speedily assumes quite a different shape. When 

 the pollen is transported by the" wind, it often happens that the flowers 

 are relatively unattractive, and the individual pollen-cells relatively small 

 and smooth. In insect-fertilized flowers, on the other hand, the flowers 

 are attractive and the pollen spiny or furrowed (Bennett). Too much 

 stress, however, must not be laid on this point. 



Pollen-grains of Gfymnospermia. 



The pollen-grains of the Gymnospermia present a modification 

 of the structure above described. They are not simple cells, but 

 produce in their cavity, even before they are discharged from the 

 anther, minute daughter cells, from one of which the pollen-tube 

 is developed, and adherent to that side of the pollen-grain where 

 the slit exists in the outer membrane. This formation is analogous 

 to what is seen in the microspores of Selaginella, which in like 

 manner produce a rudimentary prothallus (p. 425). 



According to Schacht, in Taxus and Cupressus the pollen-cell only di- 

 vides so as to form two unequal portions, of which the larger develops 

 into the pollen-tube. In Larix and Abies (fie*. 603) the pollen-grains appear 

 to consist of a central and two lateral cells of different appearance to the 

 central cell. These lateral projections are often finely reticulated, and 

 are mere vesicular protrusions of the extine. The central body is the true 

 pollen-cell, in which cell-division goes on, as in the case of Cupressus above 

 mentioned, but with the difference that three or four daughter cells of 

 unequal size are produced instead of one, the uppermost and largest of 

 these new formations being developed into the pollen-tube, which passes 

 through a rent in the extine, the other forming a kind of suspensor. 

 Strasburger dissents from this view, and says that there are never more 

 than two cells, the one marked q being- a simple fissure or rent. The 

 pollen-tube, according to him, is the result of a protrusion of the intine 

 and not of the whole cell. 



TchistiakofPs researches into the mode of formation of the pollen of 

 Conifers are remarkable as showing varying degrees of complexity and of 

 transition between the formation of new cells by division and the process 



2s 



