io8 FERNS THE ANCESTORS OF FLOWERS 



from ancient ferns the whole galaxy of modern cone- and 

 flower-bearing higher plants have been derived by a slow 

 process of change, step by step. 



Before we look further at this interesting history, let me 

 tell of a very remarkable discovery about the active little 

 swimming sperms of the fern's sexual generation. More 

 than thirty years ago, one of the ablest of living botanists 

 — Pfeffer by name — was watching with his microscope the 

 movements of the swimming fern "sperms" (see Fig. 9, B, c) 

 in the presence of one of the tiny egg-pits which, he had 

 cut off from a " prothallus," and placed in a thin film of 

 water covered by thin glass, under a high magnifying 

 power. He saw that as the sperms approached the egg- 

 pit they seemed to be drawn by some attractive force to 

 the mouth of the little volcano-like pit, and plunged with 

 great rapidity and directness into it. He came to the 

 conclusion that this must be due to the diffusion of some 

 soluble chemical substance from the mouth of the egg-pit, 

 which acted upon the distant sperms, and attracted and 

 guided them to it. He determined to find out what this 

 substance was, but the egg-pits are so minute that it was 

 not possible to collect the attractive substance, if there, 

 and to make a chemical analysis of it. He therefore 

 proceeded to attempt to solve the problem in another 

 manner. He prepared, by aid of the blowpipe, a very 

 delicate, hairlike glass tube, and breaking it into very short 

 lengths filled each little piece with a different soluble 

 chemical substance, such as might probably be produced 

 by the plant. Then he placed them one by one in a film 

 of water under his microscope, into which he had already 

 introduced a quantity of the active, swimming, screw-like 

 little sperms of the fern's prothallus. He watched to see 

 whether in any case an attractive influence was exerted 

 upon the swimming crowd of sperms. He tried a whole 

 series of possible chemical substances, such as sugar. 



