592 SIGHT. 



wither away at the approach of day. Some silenes roll up their petals in the 

 day, and expand them at night. The florets of numerous Compositse, and the 

 petals of the genus Mesembryanthemum are erect in the absence of the sun, but 

 become reflexed when acted upon by the sun's beams." " Plants of corn, in 

 which there is little indication of sleep when grown singly, exhibit that pheno- 

 menon very distinctly when observed in masses : their leaves become flaccid and 

 their ears droop at night. " " A flower removed from the shade will often expand 

 beneath a lamp, just as it will beneath the sun itself." De Candolle found he 

 could induce plants to acknowledge an artificial day and night by exposure to 

 the light of candles. Still, Prof. Lindley remarks, there must be some other cause 

 than light, because many flowers close in the afternoon while the sun shines on 

 them, and the petals of others fold up under a bright illumination. (Introduction 

 to Botany, book ii. chap, xii.) 



Just, however, as different plants require different temperatures, and the 

 protococcus nivalis flourishes and secretes in snow, so different plants require 

 different degrees of light. Humboldt, near the Canary islands, saw a marine 

 plant of a grass green brought up from a depth of about 190 feet, where the 

 light could not have been stronger than that of a candle at the distance of a foot. 

 He found several green plants growing in the dark mines of Freiberg ; but there 

 the atmosphere was peculiar, charged with hydrogen or a large quantity of 

 nitrogen ; and Senebier remarked that plants do not completely lose their green 

 colour in darkness, if they are supplied with a certain quantity of hydrogen, an 

 observation not verified in the experiments of Decandolle. (Physiologic Vegetate, 

 par M. Aug. Pyr. Decandolle. Paris, 1832. t. ii. p. 899. sq.) 



Zoophytes prove themselves sensible to light ; some by expanding or con- 

 tracting according to its intensity, or by placing themselves on the side of a vessel 

 where the light is strongest. One, the veretillum cynomorium, seeks the darkest 

 places, and contracts as soon as light is admitted to it. 



Most entozoa, living in the dark recesses of other animals and imbedded in 

 what they feed upon, require no eyes, and are not known to have them. Nor 

 the acephalous or bivalve mollusca, as the oyster, or the cirrhopodous, as the 

 barnacle, the hind part only of which protrudes from the shell. Many minute 

 polygastric animalcules seek and enjoy the light ; and, on the front of their bodies, 

 small red spots are generally discovered. Even monads, regarded as the lowest of 

 animals, have them. In other infusoria these spots are united into one. These 

 receive the impression of light only, and, as the polygastric animalcules move 

 rapidly and prey on others, perhaps not merely light, but forms are distinguished. 

 No nervous filament has hitherto been detected in such creatures: but, as they 

 have impressions from an external organ, desire and will, they must have some- 

 thing equivalent to a nervous system. This is the first form of the eye in the 

 larvae of insects when the organ begins to develope, and in the young of higher 

 animals ; and, when an optic nerve is added, this is placed behind the pigment of 

 the red spot, showing the spot to be the organ of vision. In the nais proboscidea, 

 and many of the lower tribes of annelida, an optic nerve is added to the pig- 

 ment : but nothing more is discoverable. Many of those entozoa or rather 

 epizoa which live on the external parts of others, on the skin, eyes, gills, &c. as 

 the ergasilus gibbus, which is attached in myriads to the gills of freshwater fishes, 



