588 



LECTURE XXXIV. 



the laws of physics :' for example, when the growing haulms of cereals, and other 

 shoot-axes, bend beneath the pressure of the wind and then again become erect, this 

 is simply in consequence of their elasticity, and thus of a physical property; and in 

 like manner when the cell-membrane of a turgescent cell becomes extended, or 

 contracts on the cessation of turgescence, that also is an action of a physical 

 nature. If a long-stemmed plant is laid horizontally and left free to move, the stem 

 at first bends more or less downwards, because it is flexible and elastic — that is one of 

 its physical properties ; but if we allow it to stay quietly in this position, we find after 

 several or many hours that the still growing apical portion of the stem now 

 ascends until it stands perfectly erect — and this is not a physical but a physio- 

 logical phenomenon, which only occurs in a living plant, and only there when 

 the part of the stem referred to is still growing. If possible, the accuracy of the 

 proposition expressed above comes out still more distinctly in the case of some 

 movements of the parts of plants, which in fact present great external similarity with 

 some phenomena of irritability, but are nevertheless purely physical and mechanical 

 actions, since they occur in dead though organised bodies. The upper leaves of the 

 common involucre of Carlina acaulis, a kind of Thistle, radiate outwards and down- 

 wards when the plant has long been dead, and remain in this position as long as 

 it is dry ; but on plunging such a dead flower-head in water, or letting it remain 

 in very damp air, the leaf structures referred to become directed upwards and 

 inwards, and the whole of the dried flower-head closes up ; and if desiccation again 

 follows later, it reopens— i. e. the involucral leaves again project outwards and down-, 

 wards. These changes ensue still more rapidly in the upper leaves of the involucre 

 of another composite, Myriocephalus. In both cases the movements mentioned are 

 effected by short transverse zones on the lower part of the dried involucral leaves : 

 the cell-walls on the lower side at these places swell up more strongly when they 

 imbibe water than those on the upper side, and when they dry they contract more 

 strongly than those, thus causing the movements described. 



The various movements of dry fruits and their parts, which, as has already 

 been pointed out on pp. 210-211, may sometimes be very complicated, depend 

 on similar phenomena due to the change in volume of cell-walls on the absorp- 

 tion and rejection of water. In many other cases also something similar occurs. 

 I will mention only the so-called Rose of Jericho, a plant which belongs to the family 

 Cruciferse, and which grows wild in Egypt, though it may be easily cultivated in 

 Europe. Its radiating branches, on which are situated the ripe fruits, spread them- 

 selves out on the ground, and when the whole plant is dried and dead throw themselves 

 inwards, very much as the five extended fingers of the hand are laid together in 

 forming a closed fist. If the dried plant is moistened, the branches thus rolled 

 together into a ball open out again. This process is found to occur in some other 

 plants also, e. g. an American species of Lycopodium ; and again, the hygroscopic 

 movements of the peristome of the Moss capsule (p. 1 50) depends on similar pro- 

 cesses, and so with many other phenomena in the vegetable kingdom. In all such 

 cases, however, it is not phenomena of irritability but purely physical actions which 

 come into play, such as the imbibition of water and the alteration in volume of the 

 cell-walls concerned. 



We now come to another very characteristic point, which distinguishes all phe- 



