56 Fertilization of Plants. [July, 



Some plants have appeared to offer exceptions to these general 

 principles, and have presented difficult problems for solution to 

 the phytologist. The order Asclepiadeae presents examples of 

 this kind. The pollen in this order is enclosed in sacs which do 

 not dehisce, and which have the suture towards the stigma. How 

 the pollen could act in a close bag, presented a question difficult 

 of solution. This enigma remained unanswered till about 1830, 

 when several observers noticed that the pollen emitted tubes, and 

 that they passed through the suture of the pollen sac, and entered 

 the stigma and passed to the ovarium, as in all other cases, thus 

 adding another unanswerable argument in favor of the necessity 

 of pollen in fertilization. 



Besides cases in which arrangements to bring the pollen to act 

 on the ovules, we might adduce many cases in which the ovules 

 themselves were arranged in a peculiar manner, to facilitate the 

 entrance of the tubes into the foramen, but what we have given 

 is sufficient to show the importance nature attaches to the accom- 

 plishment of this object, and she permits no obstacle to defeat her 

 design. 



Although it is very generally admitted that the pollen is essen- 

 tial to fertilization, it is not as generally admitted that it acts by 

 impregnating an ovule existing in the ovarium. The German 

 physiologists generally advocate a diffijrent theory. Schleiden of 

 Berlin, Endleicher of Vienna, and linger of Gratz, are the prin- 

 cipal supporters of the new theory, who agree in the main points, 

 but differ in some of the details. 



Endleicher says: " The pistil of the plant is not an organ that 

 can be compared to the female sexual organs of animals; it nei- 

 ther furnishes the germ or embryo destined to the propagation of 

 the species. It is simply an organ in which the embryo germ is 

 borne, to develop itself and come to maturity. The embryo is the 

 extremity of the pollen tube, which after having traversed the 

 cellular mass between the stigma and placenta, penetrates into 

 the cavity of the ovule by the mycropyle and arrives at the sum- 

 mit of the nucleus. It traverses the tissue of the nucleus, follow- 

 ing the intercellular passages, and attains the summit of the em- 



