SECT. II 



PHYSIOLOGY 



271 



light of lower intensity. Assimilatory parenchyma is also found in the 

 stem, but is uaturall}^ limited to the peripheral regions where sufficient 

 light can be obtained. Since the stereome also tends to occupy the 

 most peripheral position possible, the available space is often divided 

 between these two tissues. Stereome is also present in the leaf, 

 usually in relation to the vascular bundles ; where it is present it renders 

 the development of assimilatory tissue impossible. 



The assimilatory tissue leads by gradual transitions to the storage 

 tissue, which can occupy any available situation not required for other 

 tissues. Thus in the stem the storage tissue may have a central 

 position and in roots be peripheral. 



It is characteristic of the conducting tissues tliat the tracheal 

 elements which serve for water-conduction are associated in the 

 vascular bundle with the sieve tubes which serve for the conduction of 

 organic substances. Comparison of the distribution of vessels in the 

 leaf-blade and in the stem shows clearly that only conduction 

 takes place in the stem, while water is given off from the leaves. 

 (Cf. p. 203.) 



IV. Periodicity 



The development and growth of the organism does not proceed 

 with constant and uniform activity. In the simplest organisms 

 cell-division tends to follow on a certain amount of growth. In this 

 way two elementary organisms result, each of which proceeds to develop 

 in the same way as the parent organism. Growth and division thus 

 succeed one another in regular periodicity. In plants which have a 

 growing point the latter not only grows on uniformly, but from time 

 to time forms lateral out-growths or branches. The more complex 

 the organism is, the more obvious are periodic changes in its 

 development ; these sometimes arise from more or less recognisable 

 external causes and in other cases from purely internal changes. 

 One of the most striking of the periodic phenomena is that exhibited 

 by some plants in which after a time part of the organism dies away, 

 leaving usually only a small portion alive : from this the further develop- 

 ment proceeds. Not less remarkable is the fact that the development 

 is often arrested for weeks, months, or longer periods without the 

 capacity of development being destroyed. The three states of rest, 

 activity, and death are in the highest degree characteristic of the 

 organism. 



A resting condition or a condition of rigor results if the 

 necessary factors for growth are not present ; this has already been 

 referred to. On the other hand we see a periodic arrest of growth in 

 many tropical trees which is evidently not determined by external 

 factors or at most is connected very indirectly with such factors. 

 Although the temperature and water-supply remain continuously 

 favourable, the leaf formation does not proceed continuously, but is 



