FERTILIZATION. Ill 



107. There are three points involved in one theory which is 

 extensively adopted. First, that the tubes come in contact 

 with the sac of the amnios ; second, that the tubes do not per- 

 forate the membrane; and third, that by the action of the con- 

 tents of the tubes, an embryo is formed wit,hin the sac. These, 

 in substance, were the opinions of the first observers of the 

 action of the pollen tubes, and are still those of the English and 

 some continental botanists. But there are those who present 

 the phenomena in an entirely different light, and endow the 

 different parts with as different functions. The German botan- 

 ists, Schleiden and Endleicher, are the most prominent advo- 

 cates of the following theory, which is an abridgment of Schlei- 

 den's views. The pollen tubes enter the ovule, and pass 

 through the intercellular passages of the nucleus, and reach the 

 embryo sac, which, being forced forward, is pressed, indented, 

 and becomes the cylindrical bag which contains the embryo in 

 this first stage of its development, and which consequently con- 

 sists solely of a cell of parenchyma, supported upon the summit 

 of the axis. This bag is therefore composed of a double mem- 

 brane (except the open radicular end), viz., the indented em- 

 bryo sac, and the membrane of the pollen tube itself. In Taxus, 

 and especially in Orchis, he has succeeded in drawing out that 

 part of the pollen tube from the embryo sac which contains the 

 embryo, and that too at a considerably advanced stage. 



198. The student will observe from the above, that in Schlei- 

 den's view of the subject, the pollen tube becomes itself the 

 vessel that contains the embryo, instead of the embryo being 

 formed in the sac of the amnios; and it will also result from 

 this view, that so far from this impregnation of the embryo sac 

 coming from the pollen tube, the pollen tubes become them- 

 selves the subjects of this influence. This reverses entirely the 

 order of things, as they have been considered in all past times. 



199. After the discovery of pollen tubes, and the necessity of 

 the pollen coming in contact with the moist surface of the stig- 

 ma, in order to put them forth, it was thought that the impreg- 

 nation of the Asclepiadae and Orchideae formed exceptions to 

 the general manner of producing impregnation by their emis- 

 sion, since nature seemed to have prevented the possibility of 

 any such operation ; but more recent discoveries show these 

 plants to exhibit most beautiful examples of this arrangement. 



In the Asclepiadie, the stigma is a fleshy, five-cornered disk, 



197. How many points involved in one of the theories of fertilization? 

 "What are they?" What is Sohleiden's theory? — 198. What are the chief 

 points of this theory? — 199. What arrangement for fertilization in Ascle- 

 piadae ? Orchidese ? 



