president's addkess, 121 



80 as to make the lips of the flower part, out flies suddenly the 

 pollen. When an insect, therefore, visits such flowers and in- 

 trudes between the lips he is pelted with pollen grains, if 

 Melampyrum with white pearls, if Lamium with a golden 

 shower, if Genista with clear transparent ovals, and thus some 

 are sure to get carried elsewhere to answer the purpose of their 

 being. Indeed this is true of most, if not all, of the Scrophu- 

 lariacecE, Lahiatce^ and Legwninos^. Lastly, pollination may 

 be secured by Cleistogamic flowers where self-fertilization is a 

 necessity, as in the after-flowers of Yiolet and Pansy. 



The purpose of pollination is fertilization, that is the transfer 

 of the contents of the pollen grains through the tissues of the 

 stigma' and through the micropyle of the Oosphere, whereby the 

 Oosphere is changed into the ovule and the production of a seed 

 becomes possible. This transfer is secured by the development 

 of pollen-tubes from the intine of the pollen grains. The trans- 

 fer may be completed in a few hours, a few days, or it may 

 extend over weeks, and with some Orchids, even months. In 

 some of the latter plants the development of the ovules does not 

 even begin till pollination has taken place. These pollen-tubes 

 are usually short, but may reach a length of two inches, or in 

 extreme cases more. There may be more than one pollen-tube 

 from one pollen-grain, but the number is flxed, being always 

 the same in the same kind of pollen, though all possible are not 

 always developed. The number of pollen-tubes that start to 

 grow from one grain may be one, two, three, four, six, or even 

 more. The contents of the largest cell which develops within 

 the pollen grain passes down within the growing pollen-tube, 

 ever keeping near the growing point, till flnally it becomes 

 mixed with the cell contents of the Oosphere or Embryo-sac, 

 which completes the act of fertilization. 



The way is now clear to consider the pollen itself. An ordi- 

 nary pollen grain is a free-cell, consisting of double cell walls 

 and certain cell contents called collectively the Fovilla. The 

 outside wall is called the Extine, the inside wall the Intine. 

 These two coats may be readily separated in Thunhergia alata, 

 The fovilla consists of Protoplasm and Cellsap. The protoplasm 



