i5 



LECTURE II. 



they have been previously developed from the embrj'onal tissue of the same ; a root 

 beset with numerous lateral roots, therefore, usually possesses a naked end some 

 or several centimetres long. 



I now pass on, finally, to a short description of the proper physiological peculiar- 

 ities of the root. I confine myself again in so doing to the typical forms, more espe- 

 cially as we meet with them as subterranean roots in the land plants vvith green leaves. 

 The main purpose of these is, as already insisted upon, in addition to the fastening of 

 the plant into the ground, the absorption of fluid nourishment out of it ; and this must 

 be kept in view in judging of all the physiological properties of the root. Above all, 

 it of course depends upon this that the roots really penetrate into the substratum : to 

 this end the tap-roots, of seedlings especially, as well as those roots springing from 

 stems and destined to become further developed in the ground, are endowed with 



geotropism ; that is, the 

 growing portion, some milli- 

 metres long, lying behind 

 the growing point, reacts to 

 the influence of the gravi- 

 tation of the earth in such 

 a manner, that the grow- 

 ing point becomes di- 

 rected downwards exactly 

 vertical. This, if the root 

 had previously another direc- 

 tion, is effected by a corre- 

 sponding curvature behind 

 the growing point. If such 

 a root is placed in the 

 light first, it may be also 

 heliotropic, i. e. it becomes 

 curved at the same place 

 in such a way that the 

 growing point is directed 

 away from the source of 

 light; and both kinds of 

 reaction work therefore, in general, to the same end— that the apex of the root 

 penetrates into the ground. The same result is yet further secured, by the growing 

 end of the root being sensitive to slight pressure and to moist surfaces ; the 

 growing end becomes concave on the pressed and on the moister side, and it is 

 obvious that, under ordinary conditions of life, these two irritabilities also will 

 act like the geotropism and heliotropism of the root, and contribute to gi\e the root- 

 apex that direction by which it penetrates into the substratum. 



Concerning the origin of roots from shoot-axes, it is frequently to be 

 observed that the place of origin of the growing points is already determined by 

 external influences alone, or together with others : it is either the side lying 

 towards the centre of the earth which alone produces roots, in consequence 

 of an effect of gravity ; or, in climbing stems, it is the shaded side turned away 



Fig. 8.— Root o( Acorus calamus. Transverse section of the axial cylinder, with 

 surrounding cortical tissue; s endodermis; p pericainbiuin ; fh phloem; g groups of 

 vessels (strongly magnified). 



