444 The Physiology of Plants BOOK in 



The dependence of growth on turgor was shown by 

 Strasburger in 1882 to be sometimes very slight. He 

 pointed out that pollen grains are not turgid when they 

 put out their pollen tubes. In 1896 Copeland showed that 

 in many cases it is turgor that appears to be regulated 

 by growth rather than growth by turgor. 



The tensions that come to exist in plants as a consequence 

 of varying distribution of growth and its different rates 

 in different parts were studied by many observers. 

 Hofmeister was occupied with these problems at the opening 

 of our period. He was inclined to attribute these strains 

 to the absorption of water by the cell walls and their 

 consequent swelling, and he attached little importance to 

 the condition of turgor inside them. He added greatly to 

 our knowledge of the development of tensions during the 

 growth of different organs and to their general distribution. 

 Sachs studied the same problems a little later and broadened 

 our knowledge considerably. He showed, in 1865, that 

 growing tissues exhibit both longitudinal and transverse 

 tensions, and that they are caused in large measure by 

 the pith growing more rapidly than, and consequently 

 exerting a stretching pull on, the outer tissues. The latter 

 are consequently compressed from within outwards and 

 the more so the nearer they are to the exterior. In 1867 

 and in 1870 researches on the same subject were published 

 by Kraus, who investigated the longitudinal tensions in 

 turgid stems and petioles and the transverse strains in the 

 epidermis and cortex of various organs. He showed that 

 they are not perceptible in the youngest internode of a 

 stem, but become gradually noticeable as age increases ; he 

 attributed them to the gradual differentiation of the adult 

 elements, as they disappear when such differentiation is 

 complete. In 1867 Naegeli and Schwendener investigated 

 and explained the general principles involved in interpret- 

 ing them. Schwendener also wrote upon the fundamental 



