200 POPULAR SCIENCE MONTHLY. 



penetrate. The attenuated and undifferentiated fungi must seek the 

 shade, to escape the dangers of strong light, against which they have 

 no shield. 



The reproductive processes are particularly sensitive to illumina- 

 tion. The formation of zoospores by green felt (Vaucheria) may 

 occur only in darkness, at night, or in diffuse light, and these ex- 

 amples might be multiplied indefinitely. Many features of the ger- 

 mination of spores and the growth of protonemce or prothallia among 

 the mosses, liverworts, and ferns are determined by light. 



Perhaps the most striking reactions of plants to light are to be 

 seen in locomotor and orientation movements. 



Locomotor movements are chiefly confined to lower forms, and 

 are most noticeable in the " swarm spores," or zoospores of the algse, 

 though exhibited by spermatozooids as well. Zoospores may be seen 

 collected against the side of the vessel receiving direct sunlight, 

 while the opposite side of the vessel will be free from them. The 

 chlorophyll bodies of green cells arrange themselves similarly. The 

 latter bodies may move away from the exposed side of the cell if 

 the light exceeds a certain intensity. 



The typical plant may not move its body toward or away from 

 the source of light, but it may secure the same end by dispositions of 

 its surfaces to vary the angle at which the rays are received. This 

 form of irritability is one of the most highly developed properties of 

 the plant. Wiesner has found that a seedling of the vetch is sensi- 

 tive to an amount of light represented by one ten-millionth of a 

 unit represented by a Roscoe-Bunsen flame. The " sensitiveness " 

 to light may take one of three forms: The organ may place its axis 

 parallel and pointing toward the source of the rays, as in stems, when 

 it is said to be proheliotropic ; the axis of the organ may assume a 

 position perpendicular to the rays, which is designated as diahelio- 

 tropism; or it may place its axis parallel to the rays and pointing 

 away from the light, when it is said to be apheliotropic. Upright 

 stems are proheliotropic, horizontal leaves and creeping stems are 

 diaheliotropic, and roots and such stems as those of ivy are aphe- 

 liotropic. 



Sunlight varies from zero to the full blaze of the noonday sun, 

 and assumes its greatest intensity in the equatorial regions. The in- 

 tensity in latitudes 40° to 45° north would be represented by 1.5 

 units, and at the equator by 1.6 units. Near the equator the intensity 

 is so great that an ordinary leaf may not receive the full force of the 

 noonday sun without damage. The injury would not result from the 

 luminous rays, but from the temperatures, 40° to 50° C, arising 

 from the conversion of light into heat. As an adaptation to this 

 condition nearly all leaves have either a pendent or a vertical posi- 



