96 Comparative Aiiatomy ; or aft Atietnpl al 



scarcely a fortr.iglit elapses after the impregnat'ion of the seed, 

 before all the important parts that can precede mahirity are 

 completed. 



There are some curious changes in the form of a child before 

 it breathes, and there is also a most curious difference in the se- 

 minal leaves in a plant when compared with other plants. They 

 have no spiral wire, and of course no motion, and no midrib or 

 side-ribs : this is very conspicuous in the Mimosa se7isitiva, 

 since it is the only part that does not move if touched. It was 

 one of the proofs that enabled me to ascertain that the spiral 

 wire was the real muscle of the plant. 



There is little resemblance between the lungs of a human 

 being and the leaves of trees — nevertheless they perform for each 

 lein^ the same office ; aiid yet it is difficult to draw a compari- 

 son between them. The human lungs fill the greater part of 

 the cavity of the breast ; tliey are of a soft spongy texture, and 

 are divided into two lobes, which are separated from each other 

 by the mediastinum, and externally covered by a production of the 

 pleura : each of these is divided into tv.-o or three lesser lobes, 

 are commonly three in the right and two in the left side of the 

 cavity. There arc two series of arteries which carry blood to 

 the lungs, and it is the exposition of this blood on the surface 

 which changes its tint from a darker colour to a lighter red. 

 The pulmonary arterv and veins are not intended for the nutri- 

 ment of the lungs; but the blood in its passage through them is 

 destined to inidergo those changes, and I)econie florid by a greater 

 mixture of oxygen. Respiration constitutes one of those func- 

 tions which are proj)er!y termed vital, as being essential to life : 

 it consists of an alternate coiitraclion and dilatation of the thorax, 

 by first Inspiring air into the lung-, and then expelling it from 

 them /'y cxspiralion. When an animal has once breathed, it is 

 easy to see how the motion may be continued ; but what occa- 

 sioned the first inspiration is not so easy to discover. 



Now the difference to be discovered in the lungs of plants is, 

 that they have little inspiration of air except in water-plants, — 

 but most of the oxygen given out is taken in as water, then de- 

 composed; the hydrogen drawn off, and secreted for the seeds, 

 while the oxygen is again given out to the amosphere to purify 

 and invigorate it. The two upper cuticles of the leaf yield oxy- 

 gen, the under cuticle takes in carbonic acid gas in the night, 

 which is quickly converted into a purer air as soon as the sun 

 has power sufficient in the morning, and again returned to the 

 public stock. Many plants fold their leaves at night; but the 

 atmosphere loses not, since they give no gas at that time, but 

 the t'd'^cs and hairs of their leaves are taking in quantities of 



water ; 



