BOOK in.] Introduction. 371 



into German by Roeper and published with many improve- 

 ments and additions in 1833 and 1835 ; this was followed by 

 a work on vegetable physiology by L. C. Treviranus, 1835- 

 1838, and lastly by Meyen's ' Neues System der Pflanzenphy- 

 siologie,' 1837-1839. These works exhibit the characteristic 

 features of the period chiefly in this, that physiology finds as 

 yet no strong support in phytotomy, while the old views of 

 vital force are brought face to face with more exact physico- 

 chemical explanations of processes of vegetation. 



4. We have already pointed out the wonderful impulse given 

 to the study of morphology and phytotomy, of embryology 

 and cells about the year 1840 ; it was shown also that this was 

 due in a great measure to discarding the errors of the nature- 

 philosophy and the idea of vital force, and requiring in the 

 place of such speculations exact observation and systematic 

 induction, and how Schleiden's 'Grundziige' soon after 1840 

 vigorously met the demands of the newer time in these 

 respects, but without satisfying them by the positive results 

 obtained. The rapid progress made by phytotomy and the 

 doctrine of cells in the hands of von Mohl and Nageli proved 

 specially favourable to vegetable physiology, by making it 

 possible to follow the processes of fertilisation in the interior 

 of the ovule. The formation of the pollen-tube from the 

 pollen-grain had been observed long before 1840, and Schleiden 

 in 1837 had proposed the view that the embryo of Phanerogams 

 was formed at the end of the pollen-tube by free cell-formation 

 after it had entered the embryo-sac. But Amici in 1 846 and Hof- 

 meister in 1849 showed that this notion was erroneous, and 

 that the germ-primordium is in existence in the embryo-sac 

 before the arrival of the pollen-tube and is excited by it to 

 further development, to the forming the embryo. Similarly Hof- 

 meister's further observations on the embryology of Vascular 

 Cryptogams and Mosses left no doubt, that the spermatozoids 

 of these groups of plants discovered by Unger and Nageli 

 serve to fertilise the germ-cell or egg-cell previously formed 



B b 2 



