VEGETABLE PHYSIOLOGY. 



brought to life through the agency of the moisture. 

 Other stones equally exposed, but in dry situations, 

 have also received a clothing of these germs, but 

 circumstances not being suitable, they have not 

 been developed : give the moisture, and they will 

 immediately appear. We hew another stone from 

 the quarry, and build it into the pier of a bridge, 

 just within the surface of the water : shortly, a 

 kind of green alga will appear ; but the wet being 

 in greater abundance, and more continuous, the 

 growth will become more luxuriant than that of 

 the terrestrial wall. Instead of the simple green 

 coating, we have the addition of long filaments 

 resembling hairs (Conferva), which float and 

 accommodate themselves to the water around. 



Nature is incessantly working out vast ends by 

 humble and scarcely recognisable means. It seems 

 to be a principle that nothing shall remain station- 

 ary- or unchanged. The whole surface of our planet 

 is every instant altering in its features : mountains 

 are being washed down into the plains, rocks are 

 mouldering into soil, the sea is filling up at one 

 place and encroaching on the land at another, 

 and water-courses are constantly shifting their 

 outlines. The duty of filling up seas, ponds, lakes, 

 and rivers, is consigned to diverse means within 

 the animal and vegetable economy ; and one of 

 these is the growth of algce and other aquatic 

 plants. Take a pond of water, and shut off its 

 means of supply from rivulets and springs, and 

 then observe what an effort nature will make to 

 fill it up. The sides and bottom become speedily 

 covered with a luxuriant crop of confervas ; other 

 plants, which grow only in water, begin to make 

 their appearance, their seeds being wafted thither 

 by winds ; at length the superficial matting of 

 herbage is able to support the weight of birds ; 

 there is alternate vegetation and decay ; finally, 

 the pond is filled up, and a forest of the highest 

 order of trees may in time cover the site of the 

 original humble confervas. What, indeed, are the 

 extensive peat-mosses but lakes and pools choked 

 with vegetable matter, which remains in a half- 

 reduced condition. Thus we see that the green 

 alga which grows upon stones in the water, humble 

 and apparently insignificant as it is, performs a 

 distinct and important part in creation. 



SPECIAL PHENOMENA OF VEGETATION. 



In addition to the ordinary functions of the 

 organs, which are common to all plants, there are 

 certain anomalous phenomena which cannot be 

 reduced to regular laws, and which merit notice 

 in this place ; the most remarkable of these are 

 the occasional irritability and movements of plants. 



Irritability. The irritability of animals depends 

 entirely on their nervous system ; but as plants 

 have no nervous system, their irritability is 

 more difficult to be accounted for. The prin- 

 cipal phenomena of vegetable irritability may be 

 divided into three kinds namely, those caused 

 by atmospheric influence, those depending upon 

 the touch of other bodies, and those which appear 

 to be perfectly spontaneous. Atmospheric influence 

 occasions the closing of the leaves over the extreme 

 point of the young shoot at night, as may be 

 observed in the chickweed and several other 

 common plants. The folding of some flowers in 

 the absence of the sun, and the opening of others 

 as soon as that luminary has withdrawn its beams, 



are ascribable to a similar cause. The pimpernel 

 closes its flowers on the approach of rain. Most 

 blossoms expand during sunshine ; but the evening 

 primrose, on the contrary, will not open its large 

 yellow flowers till the sun has sunk below the 

 horizon ; and the night-flowering cereus only 

 expands its magnificent blossoms about midnight 

 Some flowers are so regular in their hours of 

 opening and shutting, that Linnaeus formed what 

 he called Flora's Time-piece, in which each hour 

 was represented by the flower which opened or 

 closed at that particular time. Solar light is the 

 principal agent in producing these phenomena ; 

 but in some cases flowers will open by arti- 

 ficial light. De Candolle found blossoms expand 

 beneath a lamp nearly as well as beneath the sun 

 itself; and the crocus and gentian, which close at 

 night, will expand as wide as possible when gently 

 exposed to the light and heat of a fire. One of 

 the most remarkable circumstances respecting the 

 effect of atmospheric influences is, that the same 

 causes do not affect all plants, and yet no peculi- 

 arity of construction has been discovered in those 

 that are so affected to distinguish them from those 

 that are not. 



The irritability produced by external touch is 

 a familiar but little understood phenomenon. The 

 movements of the sensitive plant are well known ; 

 and it is also known that if the ripe seed-vessels- 

 of the noli-me-tangere be touched in the slightest 

 manner, they will open with elasticity, and scatter 

 their contents. In the same manner the fruit of 

 the squirting cucumber (Elaterium) throws out its 

 seeds, and the pulp in which they are contained, 

 with violence, and to a considerable distance. 

 The stamens of the barberry, when touched with 

 a pin, spring forward towards the stigma, after 

 which they return to their original position ; while 

 the column of the stylidium, which includes the 

 style and stamens, and which generally hangs on 

 one side, when touched, springs with a jerk to the 

 other side of the flower. The most remarkable 

 instance of irritabil- 

 ity by contact is that 

 exhibited by Venus's 

 fly - trap (Dionaa 

 muscipula), a native 

 of North America. 

 The leaves are very 

 curiously construct- 

 ed. They have broad 

 leaf-like petioles, at 

 whose extremity are 

 two rounded fleshy 

 lobes, which form 

 the real leaf, and 

 which are armed 

 with strong spiny 

 hairs, three on the blade of each lobe, and a fringe 

 of longer ones round the margin. When an 

 insect touches the central hairs, the leaf collapses, 

 and the insect is entrapped. The leaf generally 

 remains closed till the insect is dead. In Sar- 

 racenia, or the side-saddle flower, the leaves have 

 a pitcher-shaped petiole, which forms a trap for 

 flies and other insects, as well as the leaves of 

 the Nepenthes, or pitcher-plant, but these do not 

 display irritable phenomena. 



The spontaneous movements of plants are equally 

 difficult to be accounted for with those occasic 

 by atmospheric phenomena or by external touch. 



a, Leaf of Venus's Fly-trap. 

 b, Leaf of Sarracenia. 



