PLANT HYBRIDIZATION BEFORE MENDEL 37 



"Through the substance of this elastic shell," says Kolreuter, "one 

 sees an extended net of vascular fibres which, in a few species of pol- 

 lens, is divided off into almost regular six-sided eyes, in others in an- 

 other more or less regular way." (p. 7.) 



Each such division or "eye" serves as the point of location for 

 one of the elevations or projections, in which an excretion-canal 

 terminates. Immediately beneath the outer shell is a thinner, 

 weaker, white membrane, beneath which is 



"an apparently cellular tissue, which fills the entire cavity of the pol- 

 len grain and is, as it were, the nucleus (Kern) of the latter." (p. 8.) 



It is probable that the Kolreuter idea would be better translated 

 by the word "kernel" than by the word "nucleus," with its mod- 

 ern connotations. This material or substance which, in the unripe 

 condition, is described as granular, firm and half-transparent, 

 finally at maturity passes over into a uniform, fluid, and trans- 

 parent material, which comes out of the "cellular tissue." The 

 words "cellular tissue" (zellenformiges Gewebe) must likewise 

 not be taken in the sense of "tissue-cells," but in the sense of a 

 body or mass of something, enclosed in a cell-like envelope. 

 Nothing more definite than this could possibly have been seen by 

 Kolreuter. The tube and generative cells, the only "structures," 

 visible within the pollen grain through the walls of the exine, 

 that could possibly be taken in any sense as "cellular," may have 

 been visible to Kolreuter's microscope. In this "tissue," at all 

 events, is said to be found the entire mass of the male fertilizing 

 material. 



The divisions or "eyes," thought by Kolreuter to be within the 

 "elastic shell," are evidently the more or less geometrical reticula- 

 tions on the outer surface of the exine of the pollen grain in many 

 species. The escape of the contents of the pollen grains is consid- 

 ered to be brought about by the contraction and pressure of its 

 thick outer coat. In consequence of this pressure, the contents are 

 expelled through the "excretion canals" on all sides at once. The 

 swelling of the pollen grain is presumed to take place through 

 the absorption of water. With the beginning of maturity of the 

 contents of the pollen grain, the inner coat acquires firmness and 

 elasticity and, by virtue of this, presses from all sides upon the 

 fertilizing material within, which has now become fluid, and forces 

 it into the place of least resistance, the open excretion canals. 



