AND GERMEN DURING VEGETATION. 127 



point, and not at all by any general extension of parts already formed : 

 and the new matter which is thus successively added unquestionably 

 descends in a fluid state from the cotyledons*. On this fluid, and on the 

 vegetable fibres and vessels whilst soft and flexible, and whilst the matter 

 which composes them is changing from a fluid to a solid state, gravita- 

 tion, I conceive, would operate sufficiently to give an inclination down- 

 wards to the point of the radicle ; and as the radicle has been proved to 

 be obedient to centrifugal force, it can scarcely be contended that its 

 direction would remain uninfluenced by gravitation. 



I have stated that the radicle is increased in length only by parts 

 successively added to its point : the germen, on the contrary, elongates 

 by a general extension of its parts previously organised ; and Its vessels 

 and fibres appear to extend themselves in proportion to the quantity of 

 nutriment they receive. If the motion and consequent distribution of 

 the true sap be influenced by gravitation, it follows, that when the germen 

 at its first emission, or subsequently, deviates from a perpendicular direc- 

 tion, the sap must accumulate on its under side : and I have found in a 

 great variety of experiments on the seeds of the horse-chestnut, the bean, 

 and other plants, when vegetating at rest, that the vessels and fibres on 

 the under side of the germen invariably elongate much more rapidly than 

 those on its upper side ; and thence it follows that the point of the 

 germen must always turn upwards. And it has been proved that a 

 similar increase of growth takes place on the external side of the germen 

 when the sap is impelled there by centrifugal force, as it is attracted by 

 gravitation to its under side, when the seed germinates at rest. 



This increased elongation of the fibres and vessels of the under side is 

 not confined to the germens, nor even to the annual shoots of trees, but 

 occurs and produces the most extensive effects in the subsequent growth 

 of their trunks and branches. The immediate effect of gravitation is 

 certainly to occasion the further depression of every branch, which extends 

 horizontally from the trunk of the tree ; and, when a young tree inclines 

 to either side, to increase that inclination : but it at the same time 

 attracts the sap to the under side, and thus occasions an increased longi- 

 tudinal extension of the substance of the new wood on that side f . The 

 depression of the lateral branch is thus prevented ; and it is even enabled 

 to raise itself above its natural level, when the branches above it are 

 removed ; and the young tree, by the same means, becomes more upright, 



* See the preceding Paper. 



f This effect does not appear to be produced in what are called weeping trees ; the cause of 

 which I have endeavoured to point out in a former memoir. (See above, No. IV.) 



