1.36 



RHYTHM 



If we look at the internal structure of cells during division 

 we find that certain structures (such as the Aster in the cell) 

 recur periodically. Dividing cells have recently been dis- 

 sected under the microscope with the aid of very fine glass 

 needles. They show that a rhythmic change occurs in the 

 degree of the solidity of protoplasm during cell division. 



Rhythm in Tissues 



Many tissues also exhibit rhythm. The cylinder of growing 

 cells which lies under the bark of a tree increases and multi- 

 plies during the summer and is inactive during the winter, 

 thus causing the annual rings which 

 are so conspicuous in a cross-section 

 of a trunk. This is a yearly rhythm. 

 This seasonal growth depends on 

 the alternation of summer and win- 

 ter. When the leaves fall the trees 

 cease gro^ving. But in the tropics, 

 where the conditions are practically 

 uniform throughout the year, the 

 trees make no such annual rings. 

 Yet if you transplanted deciduous ^m. 46. Transverse section of 

 trees, that is, trees that lose their an Oak-trunk, 25 years old, 

 leaves at the onset of winter, to the showing as many annual rings. 

 ., 1 1 ■ -11 i- i From Le Maout and Decaisne. 



tropics, they would still continue to 



show the annual growth, at any rate for several years, and 

 it takes some time before they lose the habit of dropping their 

 leaves in the autumn months. 



The curious plant known as the telegraph-plant, mentioned 

 in the preceding Chapter, has slender leaves which rotate 

 periodically at short intervals of time. This rotation is 

 brought about by certain cells accumulating water and 

 swelling up, thus producing pressure in a certain direction. 

 This process passes on to neighbouring cells at the base of 

 the leaf-stalk, and thus causes the whole to rotate slowly. 

 Another example of tissues in plants which show a rhythm 

 is the growing point of a young shoot. If it be carefully 

 observed it will be found that the shoot does not grow 

 straight upwards, but that as it gets longer and longer it 



