GROWTH, IRRITABILITY AND MOVEMENT 153 



succeeded by one in which reproductive structures are formed. In the autumn, 

 or earlier in some plants, growth tends to cease : the leaves and other aerial 

 organs may be lost, and a phase of dormancy opens. In annuals the death 

 of the plant occurs at this stage, the race being carried on by the seeds, 

 which themselves exhibit dormancy. It is these and many other types of 

 rhythmic change in its activities that justify the statement that periodicity 

 is a characteristic feature of the growth of the plant. 



It would appear at first sight that many of these periodic changes could 

 be ascribed without much hesitation to the direct effect of obvious climatic 

 factors. Closer examination reveals a more complex condition : more obscure 

 environmental or internal factors are then discovered to be involved. It 

 has been pointed out that the dormancy of some seeds during the winter is 

 due, not solely to the unfavourable external conditions, but also to internal 

 factors (p. 140). Again the onset of dormancy in shrubs and trees in autumn, 

 often accompanied by leaf -fall, might be ascribed to the direct effect of 

 unfavourable climatic conditions of autumn and winter. But protection 

 of the plant from the unfavourable weather will not prevent leaf-fall, and 

 will not induce the dormant buds to grow out. Several months must elapse 

 before the buds can be induced to open by merely exposing them to a favour- 

 able temperature, though more drastic treatment may " force " the buds 

 into activity. Thus buds of lilac and of other shrubs can be induced to open 

 by immersion of the plant in a warm bath of water in the region of 30 C, 

 or by exposure to ether vapour. Continuous illumination from an artificial 

 source has been seen to break down the dormancy of buds of the beech. The 

 dormancy of potato tubers can be broken by treatment with certain chemicals, 

 a method which has been used in countries where the climate permits of suc- 

 cessive crops of potatoes in one season. 



II. Irritability and Movement in Plants. 



The organs of plants normally show definite orientation. Main 

 stems grow vertically upwards, main roots vertically downwards : 

 leaves occupy a horizontal or oblique position, while the lateral 

 branches of shoot and root usually grow out obliquely. These posi- 

 tions are those in which the respective functions of the organs are 

 most efficiently performed, and the assumption of them is due to 

 the sensitiveness or irritability of plant organs towards certain 

 directional influences. Gravity is the chief of them, though the 

 direction of incident illumination is also very important for the shoot. 

 These influences act as stimuli to the organs of the plant, which 

 during their growth assume a definite orientation with regard to the 

 direction of their impact. When the organs of the plant become 

 displaced from their customary orientation, or when the direction 

 of one of these stimuli is changed, then as the result of curvatures 

 and torsions which soon appear, the organs or the younger parts 

 of them tend to resume their normal relation to the stimulus. In 



