PLANT HYBRIDIZATION BEFORE MENDEL 59 



Kolreuter then proceeds again to a detailed description of what 

 is now known to be the exine. The fire-lily {Lilium bulbiferum) 

 is taken as the type for discussion. The pollen grains of this 

 species, under "moderate magnification," appear, as he says, to 

 have a shagreen-like surface, as though covered with small pa- 

 pillae. With a "stronger magnification, one sees, instead of the 

 papillae, a net-like structure." By pressing the dry pollen grains 

 gently together between two thin sheets of mica, so that the 

 material contained in them is expelled, and bringing them under 

 the microscope, he says : 



"One will see their empty and' transparent skins entirely interwoven 

 with vascular or nerve-like threads, which are bound together, and 

 represent an irregular net with unlike angular 'eyes.' These fibres, how- 

 ever, never cut through one another, but make, even where they come 

 together, no knots, but anastomose as it were amongst one another; and 

 therein is this net-like structure wholly different from an actual net." 



(P- 253-) 



Such is Kolreuter's final description of the ridges and reticula- 

 tions on the exine, which he took for a sort of fibres penetrating 

 its tissue. 



If these fibres therefore, he says, represent sap or air-vessels, 

 the sap or air must have free access or passage from one branch 

 to another. Other species of Lilium are stated to have the same 

 structure, as also the pollen of Agave americana and many species 

 of Orchis. From observation of these and others he concluded 

 that, in a very large number of species, on the pollen, which on 

 account of its small size and other characteristics showed scarcely 

 a trace of "organic structure," there were still present similar 

 structures to those in the species indicated. The inner coat of the 

 pollen grain is described so far as it shows itself in the form of 

 the pollen tubes emerging through the germination pores. The 

 germination of the pollen grains, so far as Kolreuter observed it, 

 or was able to follow it, is described as follows. In the case of 

 Scabiosa succisa^ he gives the following 'account : The white, 

 smooth, roundish pollen grains, as soon as they are placed in 

 water, give off a great quantity of a pale, sulphur-yellow oil, 

 gradually swell with the absorbed water, and soon thereafter, 

 from three equidistant weaker places in the wall, send out, ordi- 

 narily, three conical, membranous plugs, which are immediately 

 to be distinguished from the outer, hard, and opaque shell of 



