PHYSIOLOGY 251 



roots, and from the root-pole may even produce shoots. These, however, usually 

 soon die and their place is supplied by other stronger shoots arising from the 

 shoot axis just above the roots. It is only by the most careful suppression of any 

 such developments that the shoots from the root-poles may be kept alive. In the 

 trailing shoots of such plants as the Blackberry roots may be formed beneath the 

 uninjured growing point. In so-called "weeping" trees, the formation of side 

 branches from the upper side of the hanging branches is favoured by external con- 

 ditions, but the internal polarity prevents their vigorous development, and those 

 formed soon die. In the cultivation of vines and fruit trees this peculiarity is 

 utilised to produce short-lived, fruit-producing shoots by bending over the vines 

 or training the branches of the trees in the cultivation of wall fruits. On the 

 other hand, in some cases the internal polarity is easily overcome by external in- 

 fluences. It is sometimes sufficient merely to reverse the erect thallus of Bryopsis, 

 one of the Siphoneae, to convert the former apical portion into a root-like tube 

 which penetrates the substratum and fastens itself to the grains of sand. It has 

 also been positively determined, although otherwise such cases are unknown 

 among the higher plants, that the growing points of the roots of Neottia and of 

 certain Ferns (Platycerium, Asplenium esculentum) may be converted through some 

 inherent tendency into the vegetative cone of a stem (cf. p. 47) ( 84 ). 



The correlation phenomena manifested in the formation of new organs have the 

 greatest practical importance, for the propagation of plants by cuttings or grafting 

 is based upon them. 



In artificial reproduction detached pieces of plants are made use of for the 

 purpose of producing a fresh complete plant. In many cases this is easily done, 

 but in others it is more difficult or even impossible. The favourite and easiest 

 method is by means of CUTTINGS, that is, the planting of cut branches in water, 

 sand, or earth, in which they take root (Pelargonia, Tradescantias, Fuchsias, 

 Willows, etc.). Many plants may be propagated from even a single leaf or portion 

 of a leaf, as, for instance, is usually the case with Begonias. In other cases the 

 leaves, while still on the parent plant, have the power to produce adventitious 

 buds, and, in this way, give rise to new plants (see Vegetative Reproduction). 

 Even from roots or pieces of roots it is also possible to propagate some few plants. 

 An example of this is afforded by Ipecacuanha, whose roots are cut in pieces and 

 then sown like seeds. The Dandelion possesses the same capability of developing 

 from small portions of the root, and to this peculiarity is due the difficulty with 

 which it is destroyed. 



In GRAFTING or BUDDING, cuttings from one plant are inserted in another, so that 

 they grow together to form physiologically one plant. The two parts stand in cor- 

 relation to one another, for a twig, which if planted in the soil would have developed 

 roots, when grafted on another plant forms no roots but enters into an intimate 

 relation with the stock. It adopts the roots of the latter and its buds may be said to 

 be adopted by the stock and no new organs are formed. When the affinity between 

 graft and stock is a distant one, it appears from LINDEMUTH'S observations that roots 

 may be formed from the graft even in the air. The union is accomplished by means 

 of a callus (p. 151), formed by both the scion and the adopted stock. Vessels and 

 sieve-tubes afterwards develop in the callus, and so join together the similarly 

 functioning elements of both parts. Such an organic union is only possible 

 between very nearly related plants, thus, for example, of the Amygdalaceae, the 

 Plum, Peach, Almond, Apricot, may readily be grafted one upon the other, or of 

 the Pomaceae, the Apple with the Quince ; but not the Apple with the Plum, nor 

 (as has been asserted) with the Oak. 



