Origin and Development of the Vegetable Embryo. 57 



through the canal of the outer coat, but it is very difficult to 

 trace them (as Amici also remarks) through the very narrow canal 

 of the inner coat. The pollen- tubes must not only become di- 

 minished to a third or a fourth of their former diameter, but the 

 refraction of the hght in the cylindrical cells of the inner coat 

 greatly interferes with distinct vision of its form. Some assist- 

 ance is obtained by a very slight compression of the object, which 

 is also necessary to expel air- bubbles which remain between the 

 coats and in the canal of the inner coat, when the ovule is viewed 

 in water ; and a microscope of the sharpest defining power is 

 very desirable. A magnifying power of 200 diameters suffices, 

 if its lenses be perfectly corrected. The lower end of the pollen- 

 tube reaches the rounded apex of the embryo- sac, and turns 

 toward the side to run a short distance sideways upon it. This 

 of course can only be seen when a side view is obtained ; if the 

 pollen- tube lies above or below the embryo- sac, as the observer 

 looks down upon it, he may easily imagine that it is in the interior 

 of the embryo-sac. The circumstance that the pollen-tube follows 

 the curved surface of the embryo-sac well supports the conclusion 

 that it lies upon the outer side of the latter, and runs between 

 its membrane and the inner coat of the ovule. The lower end 

 of the pollen-tube swells up considerably in a clavate form, and 

 then projects, especially at a somewhat later period, a good way 

 into the embryo-sac, probably on account of the pressure it ex- 

 periences from the coat of the ovule. The next phsenomenon is 

 a change in the interior of the lower end of the pollen-tube and 

 its inferior clavate expansion ; they no longer contain, like the 

 upper part of the pollen-tube, a clear fluid in which granules 

 are intermixed, and which has not the most distant resemblance 

 to a tissue on the eve of development, or a protoplasm destined 

 to the production of cells ; they now exhibit a coagulated, gru- 

 mous mass, of a greenish-yellow colour. That this mass results 

 from the transformation of the fluid contained in the pollen-tube 

 is evident, from the fact that in certain cases the contents of that 

 part of the pollen-tube outside the mouth of the ovule acquire a 

 similar peculiarity. This coagulated condition of the contents of 

 the lower end of the pollen-tube caused the author to feel doubt- 

 ful at this stage of his inquiry as to the real point of origin of the 

 embryo, since it seemed possible that this lower end of the pollen- 

 tube was about to become developed into it. 



One of the three cells lying at the upper end of the embryo- 

 sac now begins to grow ; in rare cases a second follows it in a 

 similar development. The protoplasm of this cell is, as will be 

 remembered, collected at the lower end ; in a short time a trans- 

 verse septum is formed; a second and two more quickly fol- 



