UNEQUAL GROWTH. - 765 



more in detail, that primary roots with strong positive geotropism, as well as secondary 

 rootlets, when growing in moderately damp air, deviate from their vertical or oblique 

 direction when there is a moist surface near them. Under these circumstances a 

 curvature concave to the moist surface takes place at the region below the apex where 

 there would otherwise be a downward curvature, the apex being by this means con- 

 ducted towards the moist surface so that it may penetrate into the moister soil or grow 

 in contact with it. The apparatus represented in Fig. 453 is well adapted to exhibit 

 this phenomenon. It consists of a zinc frame a a covered below with wide-meshed 

 network, thus forming a sieve hanging obliquely and filled with moist sawdust ff. 

 The seeds ggg germinate in the sawdust, their roots penetrating at first vertically 

 downwards into it. When the apex of a root escapes through the network into air 

 which is not too dry, it turns towards the moister surface h-m^ its geotropism being 

 thus evidently overcome'. 



Sect. 22. Unequal G-rowth ^. Our observations have hitherto had reference 

 almost exclusively to the growth of multilateral or polysymmetrical organs, such 

 as erect stems and descending roots. Organs of this kind offer the simplest example 

 of growth taking place equally on all sides. But they form only a small minority, 

 since not only a large number of primary stems like those of Hepaticoe, Rhizocarpeoe, 

 and Selaginellea?, but also by far the greater number of erect stems, and all leaves, 

 display a decidedly bilateral structure, i. e. two sides of their axis of growth exhibit 

 different characters. With this bilateral organisation is also usually connected a 

 difference in the growth of the two unequal sides, which causes curvatures and 

 hence changes in the position of the apex. The two unequal sides of bilateral 

 organs must also be acted on differently by external agencies which affect growth, 

 such as light, gravitation, and pressure. We do not attempt here to solve the 

 question of the causes which produce the bilateral structure in any particular case ; 

 it need only be shown incidentally that this structure of lateral organs (as we have 

 already seen in Book I, Sect. 27) is probably always brought about by internal 

 causes, and is independent of the action of external circumstances. This is in 

 general at once evident from the fact that the median plane of bilateral appendi- 

 calar organs has always a perfectly definite geometrical relation to the axial structure 

 which bears them, and that moreover in the dark and under the influence of slow 

 rotation round a horizontal axis, which eliminates the effect of gravitation, the 

 bilateral structure and relation to the axis remain unchanged. 



But before we proceed to the consideration of the growth of bilateral organs, it 

 must be premised that even in multilateral erect stems and vertically descending roots 

 growth does not always proceed equally and with equal rapidity on all sides of the 

 longitudinal axis ; it is much more common for first one side and then another of 

 the organ to grow more rapidly than the rest, curvatures being thus caused the con- 

 vexity of which ahvays indicates the side that is at the time growing most rapidly. 



^ [For a further detailed series of experiments on the influence of gravitation on growth, see 

 Sachs, Flora, 1873, No. 21, and Arb. des bot. Inst. Wiirzburg, 1873, Heft 3. — Ed.] 



2 A. B. Frank, Die natiirhche wagerechte Richtung von Pflanzentheilen (Leipzig, 1870). The 

 views propounded in Frank's treatise are opposed by H. de Viies in the second Heft of the Pro- 

 ceedings of the Wiirzburg Bot. Inst. 187 1, p. 223 et seq — See also Hofmeister, AUgemeine Morpho- 

 logic der Gewdchse, Leipzig, 1868, Sect. 23, 24. 



