1/9 



In the aerial parts of plants, which are accustomed to dry air, the de- 

 gree of humidity must be strikingly low if the formation of hair is to be 

 greatly stimulated as C. Kraus^ states when writing of potato sprouts. In 

 very moist air potato sprouts from the same variety are hairless, or have 

 only a few shortish hairs. Therefore, in aerial organs, it is the influence of 

 moist air in contrast to dry air which prevents pubescence. In roots, de- 

 pending mostly on water, the same effect is obtained by a continued supply 

 of water just as the influence of moist air favors pubescence. 



An extreme formation of hair on aerial and subterranean axes is there- 

 fore the result of causes acting in the same way ; the usual necessary amount 

 of water is withheld from the plants at the stage in which they are develop- 

 ing. 



In explaining the fact that greater dryness of the environment favors 

 the formation of hairs, Kraus and Mer- have cited the phenomenon that the 

 organ's growth in length is modified or arrested with the formation of 

 hairs. Both investigators are of the opinion that the material saved by the 

 arrested elongation of the cells of the axis, is utilized for the formation of 

 hairs. Besides the examples of Rhus, etc.. cited above, Heckel's^ observa- 

 tions support the theory that a scanty formation of other organs goes hand 

 in hand with a \ ery abundant development of hairs. Heckel found speci- 

 mens of Liliuui Martaijon L. and Genista aspalathoides Lam. with an un- 

 usual hair covering together with a reduction of the blossoming parts. 

 Kraus emphasizes the fact that, with the decrease of growth in length, an 

 increase in turgor takes place transversely in the whole organ (as we have 

 assumed in the development of the pith of stunted plants) whicli extends to 

 the epidermal cells and excites these to the pushing out of hairs. Vesque^ 

 like Mer and Kraus, states that increased transpiration favors hair for- 

 mation. 



Attacks of parasitic animals often excite the epidermal cells to an 

 enormous, fine growth of hair, for example, such as mites which injure the 

 young leaves with their mandibles and thus produce the so-called felty dis- 

 ease. These hair formations are described under galls. In the older my- 

 cology, such hair felts, produced by the sucking stimulus of mites, are de- 

 scribed as fungi (Erineum Pers. Taphrina Fr., Phyllerium Fr.). 



LiGNIFICATION OF RoOTS. 



The lignification of tuberous roots is due to the return to the original 

 prosenchymatous woody condition of cells in the vascular bundles which, 

 under cultivation, have become parenchymatous. The carrot, for example, 

 which serves us as food, descends from a plant whose root consists of a 



1 Kraus, Beobachtungen iiber Haarbildungen, zuniichst an Kartoffelkeimen. 

 Flora 1876, p. 153. 



- Mer, Recherches experimentales sur les conditions de developpement des poil.s 

 radicaux. Compt. rend. LXXXVIII (1879), p. 665. 



"■ Heckel, Du pilosisme deformant drms quelques vegetaux. Compt. rend. t. XCI, 

 1880. p. 348. 



■* Sur les cause.s et sur les limites des variations de structure des vegetaux. 

 Cit. Bot. Centralljl. 1884, No. 22, p. 259. 



