144? ANATOMY AND PHYSTOLOCIY OF 



effect of gravity was replaced by that of centrifiigal force. But on tlie 

 other hand, the explanation given by Knight of the mode of action of 

 gravity in determining the direction of plants mu&t be regai'ded a& un- 

 successful. In this explanation Knight started from the difterent manner 

 in which the stem and roots grow longitudinally. The root, a& is well 

 known, grows only at its extreme points. Knight believed that the half- 

 solidified substance of these points immediately followed the attraction of 

 gravitation, and curved downwards. With regard to the stem, on the 

 contrary, in which a series of internodes are undergoing elongation simul- 

 taneously, Knight thought that gravity could not act upon its already 

 formed; solid, organic substance^ but affected only its contained nutrient 

 juices, that in a stem out of the perpendicular direction those juices 

 would be drawn to the lower side, which would consequently be more 

 actively nourished, thence would grow more vigorously in the longitu- 

 dinal direction than the upper side, and so cause a curvature of the stem 

 upwards. If this explanation were correct in regard to the roots, it 

 would follow from it that the point of a root could not penetrate into a 

 fluid of greater specific gravity than its own substance ; but the experi- 

 ments of Pinot, Mulder, and Durand (" Ann, des. &c, nat. 3 /Sir,'' iii. 210 — 

 Botanical Gazette, i ) shew, that the radicles of germmating seeds pene- 

 trate into mercury, whence it is clear that the points of the roots are not 

 directly attracted downwards by gravity, but that the latter causes alter- 

 ations in the root, through which an active ctirvatiu-e downwards is 

 brought about. "We find indicated here an explanation which bears a 

 certain resemblance to Knight's explanation of the growth of the stem. 

 With reference to the latter, it is at once clear that Knight regarded aw 

 a self-evident fact, the circumstance that the curvature of the stem is a 

 consequence of its growth. But this seems in the highest degree impro- 

 bable, if we note on the one hand, that in many organs, even when they 

 exhibit no further growth (as in leaves, tendrils, <fee.), curviug movements 

 occur which depend upon a frequently very transitory expansion of their 

 cellular tissue wholly rudependent of their growth ; and remember, on 

 the other hand, that nothing is more common in stems and branches, 

 than the manifestation of a much more vigorous growth on one side, 

 giving to them a very eccentric position, ^without any curvature being 

 produced by this one-sided growth. Still less tenable must this explana- 

 tion appear when we consider that the direction upwards does not occur 

 in the stem of all plants, but that many follow a horiizontal course, and 

 that the shoots of many plants, for instance, of the Weeping Ash, have a 

 very strongly marked tendency to grow downwards, without any ob- 

 servable oceu.rrence of a difience in the mode of growtlx from that of the 

 stems gi'owing upwards. This indicates that there must exist in the dif- 

 ferent stems differences of organization unconnected with their longitu- 

 dinal growth, on which it depends that the same external conditions 

 cause in one a curvature downwards, and in the other a curve upwards. 

 That these modifications of the internal organization are based upon con- 

 ditions not very readily detected, may be concluded from the fact, that 

 the shoots of different varieties of the same plant, as of the^ Ash or Beech, 

 may behave quite differently ui this respect ; that in almost all our trees, 

 for instance in the Firs, a difference of direction exists between the 

 primary and secondary axes, and that frequently, on a sudden, without 

 perceptible external cause, the points of one or more secondary axes turn 



