678 LECTURE XXXIX. 



differences as regards the lateral shoots and roots ; the petioles especially of most 

 plants tend to direct themselves obliquely upwards, and the laminae to place them- 

 selves so that the rays of light fall perpendicularly on their surface. 



There are, hovs^ever, many other plants the shoots of which do not become 

 erect, and which Ijave no primary root penetrating vertically into the earth; 

 plants which creep horizontally on the soil, or cling close to the oblique or vertical 

 surfaces of rocks, walls, trees, &c., and extend themselves upwards or laterally. 

 There are also numerous leaf-forming shoots which, like primary roots, penetrate 

 vertically downwards into the soil, or permeate it obliquely or even horizontally, and 

 in general we find that the various organs of one and the same plant assume the 

 most various directions with regard to the horizon of the situation they may happen 

 to occupy. Frequently it also happens that an organ grows when young in a different 

 direction from that followed subsequently ; this is very conspicuous with the branches 

 of most species of Pinus, the spring-shoots of which stand erect and subsequently 

 assume slowly the horizontal position. 



It is clear that the whole aspect of a plant depends essentially upon these 

 different directions of growth of its different organs, as is at once intelligible if one 

 supposes all the roots and lateral shoots of a Fir to grow vertically upwards like 

 the main stem ; we should then have, instead of the beautiful tree-form, an ugly 

 shapeless conglomerate of organs, and the whole would be entirely incapable of 

 maintaining its existence, because none of the various organs would then be in a 

 position to discharge their proper functions. 



This very simple reflection shows at once that peculiar causes must exist to 

 compel the different organs of one and the same plant to assume different and 

 fixed directions for each organ, with respect to the horizon. One of these causes, 

 as with all vital phenomena, lies in the nature, i.e. in the internal structure of 

 the organ itself; another cause is the influence of some external force on this 

 structure of the organ, and it is in fact, as has already been pointed out, gravi- 

 tation—the gravitating force of the earth, or the general attraction of mass between 

 the earth and the minutest particles of the plant-organs — which acts on the latter 

 in such a way that they are compelled to grow in definite directions with respect to 

 the horizon, or, what amounts to the same thing, under definite angles with respect to 

 the vertical at their position. This latter is indeed only the direction of the resultant 

 of all the attractive forces of the whole earth on a definite point in the organ of 

 a plant. If it is, however, as we now know definitely, the gravitation of the earth 

 which causes the organs of a plant to assume their specifically peculiar direc- 

 tions with respect to the horizon, it follows directly that the specific differences 

 in their directions of growth can only be due to the differences in their internal 

 organization : the gravitation of the earth acts on every cell of the plant in the 

 same direction and with the same force, and if the organs are nevertheless caused 

 to react differently thereby, the cause can only be sought in a difference of the 

 internal structure of the organ itself. But as in almost all cases of irritability in 

 the animal and vegetable kingdoms, it is this very peculiarity of structure con- 

 ditioning the reaction, which is not perceptible to the senses; even the highest 

 powers of the microscope teach us nothing as to why the apex of a Fir-stem grows 

 upwards, and the tip of a lateral shoot horizontally, under the influence of gravitation, 



