37^ Introduction. [BOOK HI. 



in the female organ and to excite it to further development 

 (1849, 1851). Soon after the sexual act was observed in 

 various Algae, and these afforded the best opportunity for 

 solving by the aid of the microscope the questions which 

 experiment had still left open. Thuret showed in 1854, how 

 the large egg-cells in species of Fucus are surrounded and 

 fertilised by spermatozoids, and he even succeeded in pro- 

 ducing hybrids by fertilising the egg-cells of one species 

 with the spermatozoids of another; but it was still uncertain 

 whether simple contact of the male and female organs was 

 sufficient, or whether fertilisation is due to the mingling of 

 the substance of the spermatozoid and the germ-cell ; the 

 question was settled by Pringsheim in 1855 ; he saw the male 

 organ of fertilisation of a fresh-water alga penetrate into 

 the substance of the egg-cell and be dissolved in it, and 

 this proceeding was afterwards observed in higher Cryptogams 

 and is represented in its simplest form in the sexual act ot 

 the Conjugatae, which De Bary described at length in 1858 

 and like Vaucher regarded as a sexual process. 



When we consider to what an extent the time and power 

 of work of the most eminent botanists was devoted after 1840 

 to long and difficult observations on the minute anatomy of 

 plants, on cell-formation, embryology and the history of the 

 development of organs, we cannot wonder if other parts of 

 vegetable physiology, which require experiments on vegetation 

 in plants, were cultivated but little and by the way only ; but 

 these studies also gained firmer footing in the advance of 

 phytotomy, which supplied the physiologist with a more 

 definite idea of the organism in which the phenomena of 

 vegetative life are produced. 



The chemistry of the food of plants was one of the strictly 

 physiological subjects, which like the sexual theory was studied 

 without intermission and with considerable success in the 

 period from 1840 to 1860, but chiefly or entirely by chemists, 

 who connected their investigations into the processes of 



