220 PART III. PHYSIOLOGY. [ 49 



in a slanting position. The primary roots grow downwards through 

 the sawdust, and escape into the air (which is kept moist). At 

 first they grow vertically downwards in consequence of their 

 positive geotropism, but they soon curve upwards towards the 

 moist surface. They do this in virtue of their hydrotropic irrita- 

 bility, and it is clear that they are positively hydrotropic. 



g. Irritability of other kinds. It has been ascertained by ex- 

 periment that members of various kinds may be stimulated to 

 curvature by other causes, such as differences of temperature on 

 the two sides, galvanic currents, the flowing of currents of water, 

 and by the presentation of various chemical substances ; but these 

 various phenomena are not of such immediate importance to the 

 well-being of the plant as those which have been described above 

 in detail. 



The stimulating action of certain chemical substances (cheinio- 

 tajcis) is, however, of some importance in connexion with the re- 

 productive processes. It had been frequently observed that the 

 motile male cells (spermatozoids) of plants possessing them appeared 

 to be attracted to the female organ, fertilisation being thus en- 

 sured, but the cause of this has only recently been ascertained, 

 and only in certain cases. It appears that the female organ, when 

 it is fit for fertilisation, excretes into the surrounding water a 

 substance which attracts the male cells. In Ferns and Selaginella 

 this substance is a compound of malic acid ; in Mosses it is cane- 

 sugar. 



49. Localisation of Irritability. Among members which 

 perform movements of variation, there are many instances of well- 

 defined localisation of irritability. Thus, in the Sensitive Plant, 

 no movement ensues when the upper side of the pulvinus of the 

 primary petiole is touched, but only when the sensitive hairs on 

 the under side of the pulvinus are touched ; and, in the leaflets, 

 it is the upper side of the pulvinus which is sensitive. In Drosera, 

 the irritability of the tentacles is localised in the terminal gland, 

 In Diona3a, movement only ensues when the irritable hairs on the 

 upper surface of the leaf are touched. 



Among growing organs, tendrils offer well-marked localisation 

 of irritability. In most tendrils the lower or basal part is either 

 not at all sensitive, or is sensitive only to prolonged contact. 

 Most tendrils have their tips slightly hooked, and their irritability 

 is localised in the concavity of this curvature. The tendrils of 

 Cobcea scandens and of Cissus discolor are irritable on all sides ; 



