SECT, ii PHYSIOLOGY 249 



of climbing-roots, which in the Ivy and other root-climbers are de- 

 veloped only on the shaded side of the stem. In the Alga Caulerpa 

 the new leaf-like organs arise only on the illuminated side of the parent 

 organ. It is, on the other hand, the force of gravity which excites 

 the formation of roots on the under side of underground rhizomes. It 

 is also due to gravity that the growing points of shoots are formed only 

 from the upper side of the tubers of Thladiantha dubia, or that new 

 twigs develop, for the most part, from the upper side of the obliquely 

 growing branches of trees. Contact stimuli, on the other hand, 

 determine the primary inception and point of development of the 

 haustoria of Cuscuta. The sexual organs of Fern prothallia are always 

 developed on the side away from the light ; that is, in normal con- 

 ditions on the under side, but in case of artificial illumination on the 

 upper side. 



As a result of one-sided illumination and the stimulus of gravity, together with 

 the favouring influence of moisture, the rhizoids spring only from the under side 

 of the gemmae of Marchantia, so that eventually the two originally similar sides 

 assume an altogether different anatomical structure. 



Many adventitious formations are the result of definite external causes ; as, for 

 example, the galls induced by the stings of insects and the deposits of animal eggs 

 and larvae (cf. p. 165). 



The development of adventitious formations is especially induced 

 by MUTILATION of plants ; the same effect may result when the organs 

 or growing points though uninjured become functionless. NEW 

 FORMATIONS are in this manner produced at points from which they 

 would never have arisen on the uninjured plants. In the case of 

 Pelargoniums, Willows, and many other plants, it is possible to 

 induce the formation of roots wherever the shoots are cut. In other 

 plants, however, there seem to be certain preferred places, such as 

 the older nodes, from which, under the same circumstances, roots 

 develop. In like manner new shoots will appear in the place of 

 others that have been removed. IN THE DEVELOPMENT OF NEW 



FORMATIONS ON A MUTILATED PLANT THOSE VERY ORGANS ARISE, 

 OF WHICH THE PLANT HAS BEEN DEPRIVED. Root-less shoots 



develop first of all new roots. Roots and root-stocks deprived of 

 their shoots form first new shoots. In these processes there is 

 manifested an internal reciprocity in the formative growth of organs, 

 which has been termed the CORRELATION OF GROWTH. 



Correlation of growth is often, also, very apparent in the normal development 

 of the organs of uninjured plants. It is due to this that scales of buds are developed 

 in their special form rather than as foliage leaves. For, as GOEBEL showed, it is 

 possible by artificial means, as, for example, by the timely removal of the leaves of 

 the parent shoot of Aesculus, Acer, Syringa, tyuercus, or in the case of Prunus 

 Padus, by cutting off the upper extremity of the shoots, to induce the formation of 

 normal foliage leaves in the place of the scales. The vigorous growth which 

 ensues in the fruit and in the fruit-coverings after fertilisation and development 



