THE ACTION OF THE POLLEN. 



303 



subsequently appears. In. Gymnospermous plants (560, 573), the 

 pollen-grains grow at the orifice of the naked ovule, and immediately 

 penetrate its nucleus, just as they do the stigma in ordinary plants. 



575. Pollen-tubes may be readily inspected under the microscope 

 in many plants ; in none more readily than in the Asclepias, or 

 Milkweed, one of the plants in which this subject was so admirably 

 investigated by Mr. Brown. In that family, the pollen-grains of 

 each cell of the anther (Fig. 541) cohere in a mass ; and these 

 pollen-masses, dislodged from their cells (Fig. 542, 543), usually by 

 the agency of insects, and brought into proximity with the base of 

 the stigma, protrude their tubes in great abundance. They may be 

 seen to penetrate the base of the stigma, as in Fig. 544, and sepa- 

 rate grains with their tubes may be detached from the mass (Fig. 

 546, 547) ; but to trace their course down the style (as in Fig. 545), 

 and to their final destination, requires much skill in manipulation 

 and the best means of research. 



576. The formation of the pollen-tube commences in some cases 

 almost immediately 

 upon the applica- 

 tion of the pollen 

 to the stigma ; in 

 others it is not per- 

 ceptible until after 

 the lapse of from ten 

 to thirty-six hours 

 or more. The rate 

 of the growth of the 

 pollen-tube down 

 the style is also 

 very various in dif- 

 ferent plants. In 

 some species, a week or more elapses before they have passed 

 through a style even of a few lines in length. In others, a few 



FIG. 541. A back Tiew of a stamen of the common Milkweed (Asclepias), the appendage 

 cut away. 542. A stamen more magnified, with the two pollen-masses cohering by their cau- 

 dicles, each to a gland from the summit of the stigmatic body, to which a poUen-mass from an 

 adjacent anther is already adherent. 543. A pair of detached pollen-masses (each from a dif- 

 ferent anther) suspended by their caudicles from the gland. 544. Some of the pollen-masses, 

 with their tubes penetrating the stigma (after Brown). 545. A section through the large stig- 

 matic body and a part of the summit of one of the styles, showing the course of the pollen- 

 tubes. 546, 547. Pollen-grains with their tubes, highly magnified. (The structure of these 

 singular flowers will be more fully explained under the order AsclepiadacecB.) 



