FECUNDATION. 137 



mode of debiscence being always determined by the nature 

 of that surface. This pollenic tube penetrates the loose 

 cellular tissue of the stigma, grows downward through the 

 central portion of the style, and having arrived at its base, 

 enters the germen or ovary, in which the ovules are found 

 up to this period unfertilized. The pollen tube enters one 

 of the unimpregnated ovules through the micropyle, pene- 

 trating the tissue of the nucleus till it reaches the embryo 

 sac. Fecundation appears to be produced by the simple 

 contact of the pollen tube with the embryo sac, and the 

 imbibition by the embryonal vesicle of the contents of the 

 pollen grain through the intervening membrane, the fluid 

 contents of the two cells being thus commingled. The 

 fluid matter of the pollen grain is called fovilla, and its 

 flow through the pollen tube is easily perceived by the 

 movements of those microscopic corpuscles which it con- 

 tains. 



The pollen tubes may be readily inspected under the 

 microscope in many plants, and in none more readily than 

 in the Asclepias, or Milkweed. In that family the pollen 

 grains cohere- together in masses termed pollinia, and their 

 united tubes being protruded, are consequently of such a size, 

 as to be readily perceived by a moderate magnifying power. 



The young ovules which are attached to the placenta 

 or wall of the germen, before the flower opens, con- 

 tinue to grow until that time, but no longer, unless they 

 are acted upon by the pollen of the anthers. The neces- 

 sity for this process shows why it is that stamens and pis- 

 tils are so frequently found together in flowers, and why 

 the former surround the latter so nicely as they in general 

 do; and, in circumstances which are apparently adverse 

 to fertilization, the admirable contrivances to bring about 

 the same end. Thus, in pendulous and upright flowers, 

 12* 



