THE STRUCTURAL BASIS OF THE BODY 29 



observed in the growing parts of plants, where the root always 

 grows downwards and the stem upwards. This reaction to 

 gravity is known as geotaxis, which is distinguished as ' negative ' 

 or ' positive ' respectively, according as the plant grows in oppo- 

 sition or in obedience to the gravitational attraction. If growing plants 

 be placed on the rim of a wheel and rotated so that the centrifugal 

 force is greater than that of gravity, the stems all grow towards the 

 centre of the wheel while the rootlets grow outwards. In the same way 

 the reaction of micro-organisms to light is known as phototaxis, some 

 organisms seeking the light while others shun it. Among the primitive 

 reactions of cells perhaps the most important in the life of higher 

 animals are those grouped under the term chemiotaxis. The fertilisa- 

 tion of the ovum in the prothallus of ferns is effected by the penetration 

 of the antherozoids produced in the male organs at some little distance 

 from the female organs. It was shown by Pfeffer that the movement of 

 the antherozoids towards the ova is effected in response to a chemical 

 stimulus, probably malic acid, since he found that antherozoids sus- 

 pended in a fluid will always swim towards any locality where there is 

 a greater concentration of this acid. In the same way aerobic bacteria 

 are attracted by the presence of oxygen. If such bacteria are present 

 in a solution with an alga, on exposure of the fluid to light there is an 

 evolution of oxygen by the green alga, and a consequent congregation 

 of the bacteria round the seat of production of the oxygen. The 

 movements of the white corpuscles of the blood of the higher animals 

 are also largely determined by their chemical sensibility, and various 

 substances can be divided into (a) those which exercise positive and 

 (6) those which exercise negative chemiotactic influence on the leuco- 

 cytes. Thus the introduction under the skin of an animal of a capillary 

 tube containing a solution of substances of the first class, such as 

 peptone, tissue extracts, or the chemical products of certain bacteria, 

 leads to an accumulation within the tube of leucocytes which pass to it 

 from all the surrounding tissues. Other substances, such as quinine, 

 exert a negative chemiotaxis. Tubes filled with these, after introduc- 

 tion into the subcutaneous tissue of a mammal, will be found many 

 hours later to contain no leucocytes at all. 



THE RELATIONS OF THE NUCLEUS TO THE CYTOPLASM. The 

 universal existence in living cells of a differentiated nucleus indicates 

 that the life cycle of assimilation and dissimilation must depend on an 

 interaction between the nucleus and cytoplasm, and that each plays a 

 distinct part in the sum of the changes which make up the life of the 

 cell. The different staining reactions of nucleus and cytoplasm suggest 

 a corresponding difference in their chemical composition, a suggestion 

 which is confirmed by analysis. In the building up of protoplasm 

 proteins play an important part. They are not present, however, as 



