SECT. II THYSIOLOGY 253 



reaction may also be found in isolated cells. If a filament of Cladophora 

 is cut into single cells, each of them forms at its basal end a colourless 

 rhizoid and at the apical end a green filament (■'^^). 



This contrast of base and apex does not hold in the case of foliage 

 leaves. This depends on the fact that the regenerating leaf is not 

 included in the new formation. The latter arises as a completely new 

 plant at the base of the leaf while the leaf itself perishes. There are 

 further cases in which polarity is wanting or but slightly developed 

 in stems and roots. 



It is of especial interest to observe the effect of external influences upon the 

 position of new formations when they come into opposition to the internal dis- 

 position of the plants themselves. In this respect the beliaviour of different species 

 varies greatly. In one, the internal factors predominate, that is, the new forma- 

 tions appear quite indej)endently of exterjial conditions ; in another, the external 

 influences for the moment prevail, but the internal disposition of the plant, when 

 thus constrained for the time being, ultimately makes itself apparent and the new 

 formations never develop vigorously. A willow twig, planted in a reversed position, 

 with the shoot- pole in the ground, will produce 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. In so- 

 called "weeping" trees the formation of side branches from the upper side of the 

 lianging branches is favoured liy external conditions, but the internal polarity pre- 

 vents 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 (■'*). 



The phenomena of regeneration have great importance in horticulture since 

 they allow of plants being rapidly multiplied without the aid of seeds. 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 {Oleander, Pclargonimn, Traclesccintia, Fuchsia, Willov), 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 tlie 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 i)ieces 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 tlie root, and to this peculiarity is due the difficulty with which it is 

 destroyed. 



The polarity which is noticeable in phenomena of regeneration also 

 influences the practice of grafting. Unlike poles of a plant may 

 readily be induced to grow together, while like poles may only be 

 brought to do so with difficulty, and then do not develop vigorously. 

 As a result of such experiments, a radial polarity has also been recog- 

 nised by VOCHTING in stem and root tissue : thus, for instance, pieces 

 of a stem or root, insei'ted in a lateral incision of a similar organ, 



