BOTANICAL GAZETTE. 279 



Microscopically each filament consists of eight or ten rows of 

 very long parallel cells. On the margins are seen the outward 

 pointed tips of newly developing cells. 



A fullv grown pappus measures about 25 m. m.; that of a 

 head not yet opened about one-half this length, while one partly 

 opened is three-fourths. 



Is the mode of growth by multiplication or by the elongation 

 of the cells? It must be by elongation, for, on measuring the 

 cells of the unopened head, they are found to be only one-half as 

 long as those of the fully grown pappus. The cells of the latter 

 are about one thirty-sixth of an inch or of the entire length. 



Besides the change in the cells of the main portion, the mar- 

 ginal ones, which in the very young heads appear as barbs, be- 

 come greatly lengthened and thus give the plumose or feathery 

 appearance to the pappus 



But this elongation is not the only thing that takes place in its 

 development. Each filament, by some action, bends backward 

 until the pappus stands in all directions around the achene. 



In order to determine the manner of this action I tried the 

 following simple experiments : 



I put on the stove an achene which had not yet fully opened, 

 and therefore had the pappus still adhering. After a moment the 

 pappus began to separate and curve back, the movement showing 

 particularly at the base where it joins the little ring, the marginal 

 cells, also separated, standing out like the vanes of a feather. 

 The whole took about one-half minute, the movements being 

 quick and rather jerking. 



Taking the same specimen in which the pappus was now all 

 bent over the achene, I applied a small hair brush, wet in water, 

 to the top of the achene so that the basal cells would be moist- 

 ened first. These cells began to draw closer to the ring, slowly 

 bringing the pappus from its recumbent position. 



Again, upon moistening a piece of dried pappus with alcohol 

 and potash, the marginal cells drew quickly toward the main 

 portion. 



Afterwards, in studying specimens of Mulgedium and Lactuea, 

 I found that the pappus was joined to the achene by two or three 

 rows of small square cells. 



From these facts it would seem evident that the lower surface 

 is more susceptible to heat or moisture, contracting in the pres- 

 ence of the latter, and swelling again in the presence of the for- 

 mer; but mainly this movement is due to the small square cells, 

 for by their contraction the pappus moves downward. 



