196 Morphology and Systematic Botany under [BOOKI. 



difficult, thus not only introducing the Cryptogams into the 

 field of systematic investigation, but making them its starting- 

 point. In this way morphology not only secured a foundation 

 in exact historical development, but it assumed a different aspect, 

 inasmuch as the morphological ideas hitherto drawn from the 

 Phanerogams were now examined by the light of the history of 

 development in the Cryptogams. This was one innovation ; 

 the second, closely connected with it, was the way in which 

 Nageli made the new doctrine of the cell the starting-point of 

 morphology. Both the first commencement of organs and their 

 further growth were carried back to the formation of the separ- 

 ate cells ; and the remarkable result was to show, that in the 

 Cryptogams especially, whose growth is intimately connected 

 with cell-division, precise conformity to law obtains in the suc- 

 cession and direction of the dividing walls, and that the origin 

 and further growth of every organ is effected by cells of an 

 absolutely fixed derivation. The most remarkable thing was, 

 that every stem and branch, every leaf or other organ has a 

 single cell at its apex, and that all succeeding cells are formed 

 by division of this one cell according to fixed laws, so that the 

 origin of all cell-tissue can be traced back to an apical cell ; 

 and as early as the years 1845 and 1846 Nageli described in 

 the ' Zeitschrift fur wissenschaftliche Botanik ' the three main 

 forms, according to which the segmentation of an apical cell 

 proceeds, namely, in one, two, and three rows (Delesseria, 

 Echinomitrium, Phascum, Jungermannia, Moss-leaves). In 

 this way the separate points in the history of growth in the 

 Cryptogams were brought out with unusual clearness and 

 decision ; but on the other hand, Nageli showed in 1844 in the 

 case of a genus of Algae (Caulerpa) that the growth of a plant 

 may show the usual morphological differentiation into axis, 

 leaf, and root, when the propagative cell undergoes no cell- 

 divisions in the process of development and further growth, 

 and similar conditions were for the first time demonstrated in 

 1847 in Valonia, Udotea, and Acetabularia. Beside other 



