LITERARY NOTICES. 



847 



life of the plant. The course pursued by the 

 radicle in penetrating the ground must be de- 

 termined by the tip ; hence it has acquired such 

 diverse kinds of sensitiveness. It is hardly an 

 exaggeration to say that the tip of the radicle 

 thus endowed, and having the power of direct- 

 ing the movements of the adjoining parts, acts 

 like the brain of one of the lower animals ; the 

 brain being seated within the anterior end of the 

 body, receiving impressions from the sense- 

 organs, and directing the several movements. 



In this suggestive passage, with which 

 our authors bring their present course of 

 investigations to a close, we see opened up 

 a far-reaching prospect for the biological 

 progress of the future. For the present it 

 must suffice to have made good so much as 

 our authors have been able to report from 

 their patient study of the simpler and more 

 easily observable vital phenomena. 



A great part of Mr. Darwin's work is 

 taken up with the details of experiments 

 for measuring the quantity and direction of 

 motion in plants, both under natural and 

 artificial conditions. Direct observations 

 have been made in numerous cases under 

 the microscope, and in others use has been 

 made of delicate apparatus of various kinds. 

 Minute bits of card or tissue paper have 

 been attached to the radicles, filaments, or 

 terminals of stems, and tiny particles of 

 metal or beads of shellac have been em- 

 ployed as weights to test the power of rigid- 

 ity or of sensitiveness in the fibers of plants. 

 Pins stuck in the soil around the stem have 

 served to mark the conduct of the plant 

 when impeded in its growth or its spon- 

 taneous habits of movement. The move- 

 ments of the tenderest filaments or leaflets 

 have been made to trace themselves in lines 

 upon smoked glass. A series of diagrams 

 has in this way been worked out, and set 

 before the eye in numerous woodcuts, gen- 

 erally magnified two or three fold, showing 

 the general law of circumnutation indefi- 

 nitely modified by special conditions. The 

 differences of movement in seedling and 

 mature plants, in monocotyledons and dico- 

 tyledons, with the indications of certain 

 movements having been acquired for a spe- 

 cial purpose, arc pursued through widely 

 contrasted classes of plants. The circum- 

 nutating powers of young leaves are de- 

 scribed in thirty-three genera belonging 

 to twenty-five families, widely distributed 

 among ordinary and gymnospermous dico- 



tyledons, and among monocotyledons, to- 

 gether with many cryptogams. Here the 

 seat of movement is generally seen to lie in 

 the petiole, but sometimes both in the petiole 

 and the blade, or in the blade alone. The 

 movement is chiefly in a vertical plane ; 

 yet, as the ascending and descending lines 

 never coincide, there is always some lateral 

 movement, resulting in irregular eUipses, so 

 that the motion becomes really one of cir- 

 cumnutation. It is interesting to mark the 

 periodicity. of leaf-movement, a gentle rise 

 being observed in the evening and the early 

 part of the night, with a sinking toward 

 morning. In Dioncea and certain graminice, 

 a strange jerking and oscillatory movement 

 is to be seen under the microscope, curiously 

 contrasted with the immobility of the tenta- 

 cles of Drosera rotundifolia, which are yet 

 sensitive enough to curl inward in twenty- 

 three seconds so as to absorb a bit of raw 

 meat. The distinction of epinastic and 

 hyponastic growth according as the growth 

 takes place more rapidly in the upper or 

 lower surface of an organ, causing it to 

 bend downward or upward respectively 

 introduced by De Vries, has been illustrated 

 in the case of a number of plants. To Frank 

 is due the introduction of the useful terms 

 of " heliotropism," for the tendency to turn 

 to the light, with its correlative " aphelio- 

 ti'opism," the opposite tendency, occasionally 

 to be observed, " geotropism," for the bend- 

 ing toward the earth, and " apogeotropism," 

 expressing motion in opposition to gravity 

 or from the center of the earth. For the 

 measurement of movements, sometimes ex- 

 cessively minute, various expedients were 

 adopted. Dots w-ere made from time to 

 time upon sheets of glass placed vertically 

 and horizontally near the plant, these dots 

 being then copied on tracing-paper and 

 joined by ruled lines, arrows being added 

 to show the direction of the movement. 

 The plants were exposed to varied condi- 

 tions of light, sometimes being wholly pro- 

 tected, the light at other times being ad- 

 mitted from above or from either side. In 

 addition to the sun's rays, the electric light 

 was made the subject of experiment, with 

 results comparable with those of Dr. Sie- 

 mens. A valuable chapter is given to the 

 sensitiveness of plants to light, with its 

 transmitted efi'ects. That growth in gen- 

 eral is checked by light, which acts upon 



