704 



LECTURE XL. 



organs, lateral rootlets, side shoots, and leaves, which under the influence of the same 

 external forces assume directions oblique to the horizon or even horizontal positions 

 and at the same time strive to present their flat surfaces at right angles to the 

 strongest light. 



True to the fundamental principle vifhich we have throughout adopted, that when 

 organs react differently towards the same external influences, this must necessarily 

 be caused by a difference in their organisation, we now inquire in the first place 

 whether all qrthotropic organs are distinct in their organisation from all plagiotropic 

 ones. It can be shown that in very many cases the relations of organisation 

 are such as are indicated by the question; for all orthotropic organs are radially 

 constructed, and, on the other hand, all dorsiventral structures are plagiotropic. 

 Nevertheless it must be added that there are also many plagiotropic organs which 



Fig. ^ff].~Phaseolus muUiflorus, growing in damp soil beliind a slieet of glass, h primary root ; « lateral roots of the firet 

 order ; n n lateral roots of the second order. The roughly horizontal line denotes the surface of the soil. 



as regards their coarser anatomy appear to possess radial structure, such as obliquely 

 growing lateral roots and side-shoots which arise from orthotropic parent-organs; 

 but we may assume in such cases that certain peculiarities of structure not yet known, 

 and which are not necessarily visible by means of the microscope, determine the 

 plagiotropism, and at the same time correlations of growth very often appear to co- 

 operate also. For example, as has already been shown, the horizontal or oblique 

 growth of the branches of a Fir, depends upon the presence of the orthotropic apex 

 of the primary stem, and it may be added that a similar dependence exists also 

 between the primary and lateral roots of many plants. For example, if the tip of the 

 radicle of a Broad Bean or seedling Oak is cut away, one of the lateral roots situated; 

 immediately above the section grows not obhquely but vertically downwards; it 

 becomes orthotropic, and from henceforth replaces the orthotropic primary root. 

 Exactly the same thing occurs in the long aerial roots of tropical Aroideae. 



Between anisotropy and the correlations of growth generally, the closest 

 connection exists. The lateral structures which grow out from a strictly orthotropic 



