sect, ii PHYSIOLOGY 227 



of the correlation existing between the different parts of the plant body. This 

 polarity is particularly apparent in stems and roots, and finds its expression in the 

 tendency of every small piece of a stem to develop new shoots from that end which 

 was nearer the stem apex, while the roots take their rise from the other end. 

 Pieces of roots in like manner send out roots from the end originally nearer their 

 apex, and shoots from the end towards the stem. 



In accordance with this principle, detached pieces of stems produce new shoots 

 from their "shoot-pole," and injured roots new roots from their "root-pole." 

 This polarity, particularly investigated by Vochti^g and Sachs, makes itself 

 apparent in even the smallest pieces of stems or roots, and may, in this respect, be 

 compared to the magnetic polarity exhibited by every small piece of a magnet. 

 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 

 recognised by VOchting in stem and root tissue : thus, for instance, pieces of 

 a stem or root, inserted in a lateral incision of a similar organ, become united 

 with it, if they are so placed that the side originally outermost occupies the 

 same relative position in the new organ, but if this position is altered no such 

 union takes place. Leaves take, in respect to polarity, a special position, for they 

 are not organically included within new formations derived from them. Thus, 

 from the basal end of a leaf, an entire plant, with roots, stem, and leaves, may 

 arise, while the regenerative leaf itself gradually dies. 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 disposition of the plants themselves. In 

 this respect, the behaviour of different species varies greatly. In one, the internal 

 factors predominate, that is, the new formations appear quite independently of 

 external conditions ; in another, the external influences of 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, how- 

 ever, 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 so-called "creeping" trees, the formation of side branches from the upper side 

 of the hanging branches is favoured by external conditions, but the internal 

 polarity prevents their vigorous development, and those formed soon die. In the 

 cultivation of grapes 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 influences. It is sometimes suffi- 

 cient merely to reverse the erect thallus of Bryopsis, one of the Siphoneae, to con- 

 vert 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 

 crowing 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. 



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. 



