226 BOTANY 



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. 155). 



The development of adventitious formations is especially induced by 

 mutilation of plants. 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, Oleanders, 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, OR PREFERABLY ARISE, OF WHICH THE PLANT HAS BEEN 



DEPRIVED. Eootless 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 Gobbel showed, it is 

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

 the parent shoot of Aeszulus, Acer, Syringa, Quercus, or in the case of Primus 

 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 

 ■of the embryo in the ovule, affords another example of correlation ; for, in 

 case no fertilisation of the egg-cell occurs, all those changes which produce a ripe 

 fruit from the flower do not take place ; and, instead, another correlative process 

 is manifested by which the now useless organs are discarded. Certain plants, 

 especially those modified by cultivation, form an exception to this : in many varieties 

 •of banana, in the seedless mandarin, and in the variety of raisins known as sultana 

 etc., although no seeds capable of germination are produced, the formation of a 

 .so-called fruit is nevertheless continued. Even in these instances it is essential 

 for the formation of fruit that there shall have occurred a previous pollination of the 

 stigma, or the fertilisation of the ovules, which, however, do not mature. In some 

 few exceptional cases, however, as in the Fig, even this impetus to fruit formation 

 is not necessary. The manner of the formation of conducting tissues in plants, 

 and also their anatomical development, are regulated by correlation. From these 

 few instances it may be seen how the principle of correlation affects the most 

 various of the vital processes, even under normal conditions, and how the har- 

 monious development and function of the single members of the plant body are 

 ■controlled by it. 



The polarity manifested by plants should also be considered as a special example 



