AND FORMATION OF ROOTS. 1 57 



The tuber therefore appears to differ little from a branch, which has 

 dilated instead of extending itself, except that it becomes capable of 

 retaining life during a longer period ; and when I have laboured through 

 a whole summer to counteract the natural habits of the plant, a profusion 

 of blossoms has in many instances sprung from the buds of a tuber. 



The runners also, which, according to the natural habit of the plant, 

 give existence to the tubers beneath the soil, are very similar in organisa- 

 tion to the stem of the plant, and readily emit leaves and become con- 

 verted into perfect stems in a few days, if the current of ascending sap 

 be diverted into them ; and the mode in which the tuber is formed above, 

 and beneath the soil, is precisely the same. And when the sap, which 

 has been deposited at rest during the autumn and winter, is again called 

 into action to feed the buds, which elongate into parts of the stems of 

 the future plants in the spring, fibrous roots are emitted from the basis 

 of these stems, whilst buds are generated at the opposite extremities, as 

 in the cases I have mentioned respecting trees. 



Many naturalists* have supposed the fibrous roots of all plants to be of 

 annual duration only ; and those of bulbous and tuberous-rooted plants 

 certainly are so : as in these nature has provided a distinct reservoir for 

 the sap which is to form the first leaves and fibrous roots of the succeed- 

 ing season ; but the organisation of trees is very different, and the albur- 

 num and bark of the roots and stems of these are the reservoirs of their 

 sap during the winter^. When, however, the fibrous roots of trees are 

 crowded together in a garden-pot, they are often found lifeless in the 

 succeeding spring ; but I have not observed the same mortality to occur, 

 in any degree, in the roots of trees when growing, under favourable cir- 

 cumstances, in their natural situation. 



XIII. ON THE CAUSES WHICH INFLUENCE THE DIRECTION OF THE 



GROWTH OF ROOTS. 



[Head before the ROYAL SOCIETY, March 7, 1811.] 



I HAVE shown, in a former communication, the effects of centrifugal 

 force upon germinating seeds ; from which I have inferred that the radicles 

 are made to descend towards the earth, and the germs, or elongated plu- 

 mules, to take the opposite direction, by the influence of gravitation ; and 

 I believe the facts I have stated to be sufficient to support the inferences 



* M. Mirbel's Traite d' Anatomic, &c. &c. Dr. Smith's Introduction to Botany, 

 t See above, No. V. 



