668 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



ON STRUCTURAL ADAPTATIONS TO EXTERNAL MECHANI- 

 CAL FORCES ; AND THE DISPLAY OF THESE BY PLANTS 

 THEMSELVES. 



[Lecture delivered October 29, 1901.] 

 By Rev. Prof. G. Henslow, M.A., F.L.S., V.M.H., Sec. 



Introduction. — Professor Huxley, in speaking of Lamarckism, said 

 somewhere (I write from memory) that if it were true for the animal 

 kingdom it could not apply to the other half of the living world. This 

 is, however, precisely what it does. 



The object of the present paper is to show that plants are quite as 

 sensitive to external mechanical forces as animals ; that they not only 

 constantly display such themselves, but that they must overcome the 

 external pressures, strains and pulls, &c., to which they are subjected, or 

 they will suffer from them. 



Their capacity for doing this resides in a responsive power possessed 

 by protoplasm, which enables plants and animals alike to build up tissues 

 so as to resist possibly injurious effects from without. 



The exhibition of mechanical forces is seen in all parts of plants, and 

 at all periods of their life-history, from germination to the ripening and 

 dispersal of fruits and seeds. 



As I shall only be concerned with flowering plants, I must pass over 

 the innumerable instances of mechanical movements in microscopic 

 organisms, such as zoospores, itc, and confine myself to a few illustra- 

 tions taken from the various organs of plants, such as roots, stems, leaves, 

 flowers, and fruits. 



Germination. — I would refer the reader to Darwin's " Movements of 

 Plants " for a full account of the phenomena of motion exhibited by the 

 radicles and shoots (hypocotyls and epicotyls) of germinating plants. 

 The mechanical forces therein displayed are of the utmost importance. 

 Thus, " circumnutation," or "bowing around," as the term implies, of 

 the radicle, enables it to find a point of least resistance where it can pene- 

 trate the soil. The energy of growth enables it to thrust its way in with 

 great force. Darwin roughly measured this energy and found that the 

 tip of the radicle of a bean could exert a force of upwards of a quarter of a 

 pound in twenty-four hours. In another case it was 8 lb. 8 oz. ; but it 

 was, as he says, probably much greater than that. Moreover, even soft 

 fungi have been known to raise, not only the hardened asphalt on a path, 

 but even the pavement of a street in Basingstoke, and a cemented 

 hearthstone of a kitchen. 



Similarly the upward growth of the stem of a seedling underground, 

 aided by circumnutation, readily upheaves a considerable weight of earth, 

 and overcomes the resistance of the great pressure of the soil upon it. 



When the stem has reached the surface and begins to grow erect in the 

 air, it is perfectly obvious that it is constantly subjected to mechanical 

 strains, especially of gravity or the attraction of the earth, which is always 



