506 



PHYSIOLOGY OF GROWTH. 



We entirely cover the inside of the cylinder with strips of wet 

 blotting-paper ; these must dip into the water at their lower end. 

 The mouth of the cylinder is covered with a sheet of glass, We 

 may now perform our experiments at very different times of the 

 year, and will first see what results we obtain with willow twigs 

 in February or March. If, at this period, we suspend in our 

 cylinder (see Fig. 169) pieces of willow stem (Salix viminalis or 

 Salix fragilis) about 200 mm. in length and 12 mm. in diameter, 

 beset throughout their length with buds 

 as nearly as possible of the same size, the 

 morphological apices of the stems being 

 directed upwards, their basal ends down- 

 wards but not dipping into the water, 

 then the buds soon begin to burst, and also 

 the primordia of roots concealed beneath 

 the cortex begin to develop.* In the moist 

 air of the culture cylinder, at a sufficiently 

 high temperature (say 20 C.), and in dark- 

 ness, the pieces of stem produce in the 

 course of three or four weeks vigorous 

 shoots and roots. We now find, however, 

 that shoots only develop from the buds 

 at the apex, while roots break forth from 

 a very considerable part of the surface. 

 In many experiments, however, which I 

 conducted, I invariably found more or less 

 clearly, my results here harmonising with 

 those of Vochting, 1 that the roots increase 

 in number and length towards the mor- 

 phological base of the pieces of stem 



(see also Fig. 116). If in July we cut out from the middle of 

 a vigorous current year's willow stem pieces say 200 mm. in 

 length, remove their leaves and suspend them in moist air in 

 cylinders, as described, shoots develop at their upper ends. 

 As to the development of roots, that is confined in the young 

 pieces of stem, in contrast with older ones, to their morphological 

 base. We also suspend still older or younger pieces, in March or 

 July, with their morphological apex directed downwards and their 

 morphological base upwards. It again appears that at the apex 



* It must be emphasized that the root primordia are uniformly distributed 

 under the cortex throughout the entire twig. 



FIG. 189. Glass cylin- 

 der in which is suspended 

 a sprouting piece of a 

 willow twig. 



