314 



FERTILIZATION. 



prolonged downwards into the style, gliding along the interspaces 

 between the very loosely disposed cells 

 of the conducting tissue, where it finds 

 abundant moisture, and at length reaching 

 the placenta or some other part of the in- 

 terior of the ovary. This prolongation 

 into a tube, often many hundred times the 

 diameter of the pollen-grain, is a true 

 growth, after the manner of elongating 

 cells (35, 97), except that it seldom if 

 ever ramifies, nourished by the organiza- 

 ble moisture of the style which it imbibes 

 in its course. Now the orifice of the 



ovules, or a projection of the nucleus beyond the orifice, is at this 

 time brought into contact with, or proximity to, that portion of the 

 walls of the ovary from which the pollen-tubes emerge ; and a 

 pollen-tube thus reaches the nucleus, in which the nascent embryo 

 subsequently appears. 



574. The pollen-tubes may be readily inspected under the mi- 

 croscope in many plants ; in none more readily than in the Ascle- 

 pias, 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. 420) cohere in a mass ; and 

 these pollen-masses, dislodged from their cells (Fig. 421, 422), usu- 

 ally by the agency of insects, and brought into proximity with the 

 base of the stigma, protrude their tubes in great abundance, and 

 of a size which renders them visible with a very moderate magni- 

 fying power. They may readily be seen to penetrate the base of 

 the stigma, as in Fig. 423, and separate grains with their tubes 

 may be detached from the mass (Fig. 425, 426) ; but to trace 

 their coarse down the style (as in Fig. 424), and to their final des- 

 tination, requires much tact in manipulation and the best means of 



575. FnatiM tf fk falfjl Before the pollen-tube 

 the wrote, the nucleus of die latter eih ibrts a carity in its 

 towards the apex. In ihe Mistletoe, this cavity would seem to be a 



