On the Noxiriiliment produced lo the Plant ly its Leaves. 9 



the midrib of the leaf, in a manner too plain to be denied: before 

 the flower has at all appeared at the exterior of the leaf, it may 

 be seen under the skin in knots, and down the midrib of each 

 leaf; and if that is divided, they may be taken, and, if placed in 

 the microscope, show the seeds and baskets. It is still more 

 evident in tlie xylophylla. 



I now turn to the cuticle of water plants. 1 showed in a for- 

 mer letter, that there are two sorts of water plants, those which 

 lie on the surface, and those which rest below it : sometimes 

 they are joined in the f&xat plant, as in the water lilies. It is 

 the latter sort which rise each day to the top of the water to 

 fill themselves with air, nor could they carry on their interior 

 mechanism but for this resource. The water ranuvculus and 

 many of the potumogtlo?}.'!, if watched, will be seen, though often 

 loaded and twined together into a real weight, to rise by de- 

 grees \vith a sort of undulatory motion, till the whole has ap- 

 peared for a few seconds above the surfoce of the water. I have 

 taken them in the very act of risins", and found them almost void 

 of air : but when, after giving them a little time, I have allowed 

 the part to rise, and sink again, and then taken them quickly 

 from the water, I have always fouiid tlieni loaded with air, 

 one large bubble being constantly attached to each bud, and 

 absolutely necessary to its existence ; for, if taken away by pres- 

 sure, the bud directly decays. I have long been convinced that 

 air is as indispensable to vegetable as to animal life. All the trials 

 that have been made to oblige plants to grow in an exhausted 

 receiver prove thit, but do not try the experiment half so well 

 as pressing those bubbles of air out of water plants, because the 

 vacuum is not to be maintained perfect enough or sufKciently 

 long for a plant to shoot. But in order to be assured whether 

 the buds could live under water if each was robbed of its bubble 

 of air, I took some ramivadus aqnaticus to grow in a deep tub ; 

 then seized a piece about an inch or two ; and after pressing 

 away the bubble, I tied it at each end, so tiiat the air could not 

 return ; but the bubble would not leave the bud without breaking 

 the skin which contained the air, so completely was it linked to 

 that which it vvas to support. I replaced the piece directly in 

 water, but the buds were dead and the line of life black in less 

 than an hour. It is inconceivable how soon a fresh-water plant 

 dies, if taken from its element ; quite as quickly as a fish, and 

 decays much sooner, for it grows putrid almost directly. May 

 it not be the same thing that kills it, which kills the fish ? viz. 

 the want of inward air to sustain the excessive weight of the 

 outward atmosphere. There is undoubtedly a very strong simi- 

 litude in their interior formation. But what brings on that 

 immediate decomposition P There is certainly a very curious 



fact 



