102 OF THE ROOT, 



drought. We have already mentioned, /;. 43, the ac- 

 quisition of a bulb in Phleum prateme^ll) Engl, Bot. t. 

 1076, whenever that grass is situated in a fluctuating 

 soil, by which its vital powers are supported wlule the 

 fibrous roots are deprived of their usual suppHes. In 

 this state it becomes the Phleum iiodvsiini of authors ; 

 but on being removed to a thoroughly wet soil, it re- 

 sumes the entirely fibrous root, and luxuriant growth, 

 of Ph. pratense. I have also found yllopecurus genicula- 

 tiis, t. 1250, (an aquatic grass, whose root is naturally 

 fibrous and creeping,) growing with an ovate juicy bulb 

 on the top of a dry wall. This variety has been taken 

 for the true A. Bulbosiis, t. 1249, which has always 

 bulbs even in its native marshes. We see the wisdom 

 of this provision of Nature in the grasses above mention- 

 ed, nor may the cause be totally inexplicable. When a 

 tree happens to grow from seed on a wall, it has been 

 observed, on arriving at a certain size, to stop for a 

 while, and send down a root to the ground. As soon 

 as this root was established in the soil, the tree continu- 

 ed increasing to a large magnitude.* Here the vital 

 powers of the tree not being adequate, from scanty nou- 

 rishment, to the usual annual degree of increase in the 

 branches, were accumulated in the root, which tl^erefore 

 was excited to an extraordinary exertion, in its own nat- 

 ural direction, downward. There is no occasion then to 



(17) [Common Herds grass with usj or Timolhy tjrass.] 



* A particular fact of this kind concerning an ash was coin ■ 

 municated to me by the late Rev. Dr. Walker of Edinburgh. 

 See also Trans, of Linn. Soc. v. 2. 263. 



