CHAP, iv.] Origin of Tissues. 315 



aleurone-grains. To starch-grains, which had been frequently 

 examined, by Payen especially, Na'geli devoted an investiga- 

 tion at once comprehensive and profound, and obtained results 

 of extraordinary value ; these were given to the world in an 

 exhaustive work published in 1858 under the title 'Die 

 Starkekorner,' and form an epoch not in phytotomy only, 

 but in the general knowledge of organised bodies. By 

 the application of methods of research unknown before in 

 microscopy, Nageli arrived at clear ideas of the molecular 

 structure of the grains, and of their growth by the introduction 

 of new molecules between the old ones. This theory of intus- 

 susception founded on the observation of starch-grains derives 

 its great importance from the fact that it served directly to 

 explain the growth of cell-membrane, could be applied generally 

 to molecular processes in the formation and alteration of 

 organic structures, and accounted for a long series of remark- 

 able phenomena, especially the behaviour of organised bodies in 

 polarised light. Nageli's molecular theory is the first successful 

 attempt to apply mechanico-physical considerations to the ex- 

 planation of the phenomena of organic life. 



While men of the highest powers of mind were devoting 

 themselves to the solution of these difficult problems, the study 

 of tissues was not neglected in the years after 1840, and here 

 too it was Nageli who gave the chief impulse and the direction 

 to further development. In the periodical which he published 

 in conjunction with Schleiden he had already (1844-46) given 

 an account of some searching enquiries which he had made 

 into the first processes in the formation of vascular bundles 

 from uniform fundamental tissue ; in the Cryptogams he 

 observed the production of the tissue of the whole plant from 

 the apical cell of the growing stem, and this discovery, still 

 further pursued by Hofmeister especially, has given rise during 

 the last twenty years to a copious literature, which has been 

 of service to the theory of the formation of tissues, to 

 morphology, and consequently also to systematic botany. 



