250 BOTANY i AKT i 



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, especi- 

 ally 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 

 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. According 

 to MULLER THUKGAU ( 63 ) the formation of seeds in grapes exerts an influence on the 

 form, quantity, and quality of the succulent portion of the berry. In some few 

 exceptional cases, however, as in the fruits of the Fig and Gherkin and the seeds 

 of Cycads, even this impetus to fruit formation is not necessary. The manner of 

 the formation of conducting tissues in plants, and also their anatomical develop- 

 ment, are regulated by correlation. From these few instances it may be seen how 

 the principle of con-elation affects the most various of the vital processes, even 

 under normal conditions, and how the harmonious 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 

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

 polarity is particularly apparent in steins 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 VOCHTING and SACHS, is supplemented accord- 

 ing to GOEBEI. by the nutritive current setting towards the wounded surface ; it 

 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 recog- 

 nised by Yik'HTlNu 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 for the moment prevail, but the internal dis- 

 position 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 



