278 



WENHAM, ON THE CIRCULATION OF THE SAP 



The thickness of the leaf is composed of two layers of cells, 

 irregular botli in form and position, as shown in fig, 2. The 





margin of the leaf consists of a single layer of cells of great 

 transparency ; it is in these that the remarkable phenomena 

 accompanying the circulatory movement are best seen. To 

 observe this satisfactorily a good eighth is necessary, having 

 an aperture of from 1 20° to 1 30° ; if it extend beyond this 

 the object will be less perfectly shown, on account of the 

 close approximation of the front lens, and the difficulty of 

 adjusting for thickness of cover. For illumination I prefer 

 the achromatic condenser with a series of stops, and for con- 

 taining the object, a compressor having thin glass both over 

 and under the object. The best leaves for examination are 

 those which have slightly changed colour from age ; the 

 young and vigorous specimens oftentimes displaying the cir- 

 culation but very feebly. 



Upon first seeing the object under these conditions, it 

 appeai'ed to me that there was something very remarkable in 

 the structure of the immediate surface of the walls of those 

 cells in which rotation could be seen : I immediately removed 

 the microscope into direct sunlight. As thus illuminated the 

 whole interior of the cell appeared to be lined with cilia, each 

 developed in a most distinct manner, and altogether exhibiting 

 the wavy undulating appearance usually caused by ciliary 

 motion. The movement of the green chlorophyll granules 

 also tended to favour this deception ; for by the action of the 

 supposed cilia they were occasionally collected together in a 

 mass at one end of the cell, and the particular manner in which 

 the preceding ones were again disentangled, one by one, at 

 the point of least resistance, seemed to be due to the mecha- 

 nical or sweeping power of the cilia. 



If the existence of cilia in plants of this description could 

 be established, it would no doubt serve to explain many 

 obscure points in vegetable physiology ; but subsequent ob- 

 servation luis shown me that the appearance of these in the 



