CHAP, iv.] Origin of Tissues. 315

aleurone-grains. To starch-grains, which had been frequently examined, by Payen especially, N�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, N�geli 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 N�geli 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.