CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT. 755 



the Daffodil are negatively geotropic when in the bud, but they 

 become diageotropic as they open. 



/. Irritability to Differences in the degree of Moisture in the sur- 

 rounding Medium (Hydrotropism). 



Irritability of this kind is especially characteristic of earth-roots 

 which possess it in a high degree. It can be readily demon- 

 strated by a well-known experiment. Peas or Beans are made to 

 germinate in a sieve full of damp sawdust, the sieve being sus- 

 pended in a slanting position. The primary roots grow down- 

 wards through the sawdust, and escape into the air (which is 

 kept moist). At first they grow vertically downwards in conse- 

 quence of their positive geotropism, but they soon curve up- 

 wards towards the moist surface. They do this in virtue of their 

 hydrotropic irritability, and it is clear that they are positively 

 hydrotropic. 



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

 ment that members of various kinds may be stimulated to curva- 

 ture 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 {chemio- 

 taxis) is, however, of some importance in connexion with the 

 reproductive 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 ensured, but the cause of this has only recently been as- 

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

 organ, when it is fit for fertilisation, excretes into the surround- 

 ing 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. 



§ 13. 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 hail's 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, 



