68o 



LECTURE XXXIX. 



on the root, W, only here the upper and lower sides behave in a manner exacdy the 

 opposite from the above. 



The pordon of the plumule 6" here considered is again represented in another 

 manner at B, the case here selected being, it is true, not the most usual, 

 though it is the most instructive: the upper side oo shortens to do, while the 

 under side elongates much more considerably, as shown by u'u'. The curvature 

 which necessarily results from the shortening and lengthening is, however, not 

 indicated in this diagram, so that the relative lengths may come out better on the 

 straight lines. It is easily intelligible that it would also suffice for the purpose 

 of curvature if the upper side oo retained its original length, while tiu elongated, 

 and in fact the Hne o o might elongate if only tl li sho\\^ed a greater elongation : 

 in this case also such a curvature would result that the upper side oo becomes 

 concave, and the lower side u u convex. All these three cases may actually be 

 observed in the upward curvature. Exactly the same applies to the diagram C, 



which represents that part of a root in which curvature is chiefly taking place, only 

 that all the matters relating to the upper and lower sides are converse to those in B, 

 bringing out sharply the meaning of the expressions positively geotropic and 

 negatively geotropic : B represents the process of growth in a negatively geotropic 

 organ, C that in a positively geotropic one. 



We might now suppose Fig. A to represent a germinating non-cellular plant, 

 such as Vaucheria : in this case the whole structure is a continuous utricle, the 

 cellulose-wall of which is indicated by the outlines. Our observations here simply 

 refer to two equally long portions o o and u u of the upper and lower sides of the 

 utricle. We might, however, also assume this utricle to be subdivided by more or 

 less numerous transverse septa into a series of cells, and that the pordon o o, u n, 

 represents one of these cells, and it is intelligible that the above considerations would 

 not be essentially affected thereby. Finally, we may also assume that within the 

 oudine of Fig. A not only transverse walls but also longitudinal walls were present, 

 and that thus the whole of the space included by the outline consisted of more or less 

 numerous layers of cell-chambers. Even in this case the diagrams B and C would 

 again serve to represent two individual cell-chambers, and we might suppose the cell 

 B just as well situated on the lower side of the shoot 6" as on its upper side, and 

 exactly the same would hold good of the diagram C with respect to the root W. 



