POLARITY. 



485 



reactions of both parts towards external influences, or, what is the same thing, from 

 their irritabiUty; and the phenomena of irritability which appear in all parts of 

 the plant without exception, will only become clear and comprehensible in all respects 

 when the polarity of the organ is taken into account also. That a tree developes its 

 crown in the air and its root in the earth is ultimately to be referred to the polarity 

 of the root-end and shoot-end already present in the fertilised oosphere, by means 

 of which the capacity to re-act towards light, gravitation, &c. is so distributed that 

 the upright position of the tree finally results from it. Of course this can 

 only become intelligible when the phenomena of geotropism and heliotropism 

 are more closely studied. What has just been stated is only for the purpose of 

 pointing out that it is no mere play- 

 ing with empty terms when we speak 

 of polarity in the plant ; on the con- 

 trary, polarity denominates its whole 

 life and growth. 



Those plants which creep on 

 the surface of the earth, or float 

 horizontally on water, usually under- 

 go a change of their polarity on 

 further development. This may 

 take place to such an extent that 

 the original root perishes, while 

 roots make their appearance on 

 the under or shaded side of the 

 forward-growing shoot, the upper 

 side of the same shoot producing 

 leaves and other organs. In this 

 way arise dorsi-ventral plants, the 

 shoot-axes of which possess a ven- 

 tral side whence the roots spring, and a true shoot-side (as one may best term 

 it) or upper, light side. There thus appears a polarity transverse to the axis of 

 growth of the shoot — a polarity which elsewhere exists in the direction of the 

 shoot-axis itself. However, I shall return to this important point, apart from 

 which the life of very many plants remains a puzzle. 



As a preliminary, it is necessary to make ourselves acquainted somewhat 

 more in detail with respect to the distribution of the relations of organisation 

 on the transverse section, or rather around the axis of growth. Unfortunately 

 there is no satisfactory word to designate what is here referred to ; for lack of a 

 better the term Laterality may be employed'. It includes the two cases of radial 



Fig. 3=1.— Transverse section of the flower-scape of Allium schceno- 

 pyasum (X about 30). «epidermis; ch chlorophyll cells, r colourless 

 parenchyma of the cortex ; >n parenchyma of pith ; gg' vascular bundles ; 

 sy ring of sclerenchyma. 



1 The mutual relations of the parts of a plant, designated Laterality in the text, correspond 

 in the main to what was formerly termed the symmetry of plants. After a very valuable treatise of 

 Möhl's {Vermischte Schriften, 1845, p. 12) botanists for more than twenty years did not concern 

 themselves with these very important matters, until I again took up the subject in the second 

 edition of my 'Text-book,' 1870, p. 26, though proceeding from other points of \äew. In my treatise 

 Über ort hotrope und plagiotrofe Pflanzentheile' (Arb. des bot. Inst. B. II, p. 226) I then brought 

 into notice the intimate connection between radial structure and orthotropic growth, and between 

 dorsi-ventral structure and plagiotropic growth. 



