276 Mr. A. Henfrey on the Progress of Physiological Botany : 



tral cells parallel with the surface of the leaf, and the process ex- 

 tends outward from the central to the lateral cells according to 

 the specific peculiarity of the plant, but it is always the central 

 cell which first divides. 



In the Lycopodiacea and Equisetacea the leaves also grow in 

 length and breadth by the development of their apical and mar- 

 ginal cells. The Phanerogamia follow the same law. In an im- 

 perfect leaf, the cells at the border and apex are full of the 

 homogeneous mucilage (protoplasm), while in the others it is 

 already transformed into yellowish or greenish granular matter. 

 Sometimes the formation of the septa may be observed in the 

 marginal cells. In thin leaves the increase in length by the di- 

 vision of the apical cell may frequently be observed during the 

 growth of the plant. 



The fact that the leaves of Phanerogamia grow at the apex and 

 borders and not by the base, is most easily seen in compound or 

 much-divided leaves. As a general rule the lateral axes shoot out 

 from the main axis in succession from below upwards ; in like 

 manner grow the tertiary axes (when present) from the secondary. 



In Astragalus (which is figured by the author) it is shown 

 that the uppermost leaflets are the youngest, the lowest the old- 

 est and largest. 



In Utricularia the growth of the leaf originally and of the di- 

 visions subsequently, may be seen to occur by continual deve- 

 lopment at the apices of the main axis and the divisions. In 

 Myriophyllum however was found an exception, the upper lobes 

 of the leaf being formed first. 



The thickness of the leaf, the various inferior layers of the 

 epidermis and the parenchyma depend on another mode of 

 growth, which Nageli calls, in opposition to the peripherical cell- 

 development, cell-development in every direction [allseitige Wachs- 

 thum). Three forms of this occur: 1. It is either absent or 

 merely follows at a little distance the development of the apical 

 cell from below upward and soon ceases. In this form the de- 

 velopment of the cells often ceases in the lower part of the leaf 

 before it is complete at the borders. Utricularia is an example. 

 2. The development in every direction occurs simultaneously in 

 all parts of the leaf, which completes its peripherical growth very 

 rapidly. The growth in this form usually ceases in all parts 

 about the same time. 8. The development in every direction 

 begins, after the rapid completion of the peripherical growth, 

 either only or at all events principally on the upper part of the 

 leaf and extends downward. It ceases at the base last. 



There is also sometimes an abnormal growth in every direction, 

 which occurs either in particular cells or in the whole tissue, and 

 does not appear to be subject to any laws. 



