104 



THE AMERICAN MONTHLY 



[June, 



jective of two inches equivalent focus, , 

 which, in practice, is the most satis- 

 factory power for the purpose. This 

 microscope is used to examine an 

 image of a diaphragm, formed by the 

 ocular to be measured. The exact 

 size of the diaphragm and its dis- 

 tance from the ocular being known, 

 the size of the miniature image 

 formed by the ocular can be readily 

 measured, and a simple calculation 

 then gives the magnifying power. 



In the figure, AB represents the 

 examining microscope ; C is a sta- 

 tionary stage having a micrometer 

 ruled in lines o.i mm. apart. The 

 tube with draw-tube, GD, carries the 

 ocular to be tested at G, and the dia- 

 phragm of known size at D. The 

 shape of the diaphragm-opening is 

 shown at Zr,Zr'. By means of the draw- 

 tube the distance GD can be changed, 

 but it is usually made ten inches from 

 the diaphragm of the ocular to the 

 end of the tube. The v/idtli of the 

 aperture at D, in the instrument used 

 by Mr. Bulloch, is 6.5 mm. Directing 

 the instrument toward the light, an 

 image of the aperture will be formed 

 by the ocular, and can be focussed 

 upon the micrometer at C. Then 

 the size of the image can be read off 

 in the microscope, and the difference 

 between the size of the image and 

 the actual size of the diaphragm in- 

 dicates the power of the ocular. Sup- 

 pose the image just covers eleven 

 divisions on the micrometer, the dis- 

 tance being ten inches as above, then 

 65 divided by 11 is about 6, which is 

 the magnifying power in diameters. 



A Bausch & Lomb periscopic ocu- 

 ler " C," measured in this way gave a 

 power of II diameters ; a Tolles ]/2- 

 inch solid, 16}^, and a Gundlach ^- 

 inch, 43. 



Pollen-tubes. 



About half a century ago, Amici 

 observed that the pollen-grains pro- 

 duced tubular appendices by the ab- 

 sorption of moisture on the stigma, 

 and came to the conclusion that 



these tubes descended from the 

 stigma through the style to the pla- 

 centa. Several other writers after 

 him made the same observation, and 

 further, brought the pollen-tubes in 

 connection with the process of ferti- 

 lization of the ovules. 



M. Brogniart was of the opinion 

 that the pollen-tubes, after a shorter 

 or longer penetration into the stig- 

 matic tissues, expand on the accu- 

 mulation in them of the contents of 

 the pollen-grains, that the membrane 

 of the tubes bursts, and the fovilla is 

 thus scattered amongst the papillse 

 of the stigma. 



M. Tulasne stated, in 1849, that he 

 had seen the end. of the pollen-tube 

 come in contact with the membranes 

 of the embryo-sac, without producing 

 there a depression or a strong adhe- 

 sion. 



The theory of the fertilization of 

 the ovules by means of the tubular 

 appendices of the pollen-grains des- 

 cending through the style to the 

 ovarian cavity, and thence seeking 

 their way to the foramen of the 

 ovules, is now universally adopted by 

 authors, and, as it appears, mostly 

 without personal examination or 

 observation ; some of them at- 

 tempt to explain the manner in 

 which the pollen-tubes overcome the 

 difficulties they find in their way to- 

 wards the foramen. Lindley, in 

 speaking of the fructification of the 

 orthotropous ovules of the rockrose, 

 states, on the authority of M. Bro- 

 gniart, that the pollen-tube does not 

 follow the placenta till it reaches the 

 ovule, but quits the style at the top 

 of the cavity of each cell, and thence 

 lengthens in the open space inside 

 the ovary till it reaches the foramen 

 in the end of the ovules. Mr. Det- 

 mar has published more recently also 

 a paper on the " Course of the Pollen- 

 tubes," a synopsis of which was pub- 

 lished in the April number of 1881 

 of the Journal of the Royal Micro- 

 scopical Society, of London. I fol- 

 lowed his description, having prepa- 

 rations of the ovaries of most of the 



