CHAP, x The Problems of Growth 445 



mechanical principles involved in the construction of the 

 plant-body in 1874, as we have noticed in another 

 connection. 



An indirect effect of the tensions in plants, resulting from 

 the juxtaposition of turgid cells and pitted vessels, is the 

 formation of thyloses, which result from the bulging of the 

 former through the pits in the walls of the latter, followed 

 by proliferation of the intruding tissue. These thyloses 

 were described in 1868 by Reess. In 1882 Strasburger 

 showed that such internal pressures sometimes cause the 

 obliteration of the cavities of the softer vessels which abut 

 on the turgid parenchyma notably in the case of the older 

 sieve-tubes of Pinus. 



Sachs showed in 1879 that the assumption of fixed posi- 

 tions, erect, or inclined at an angle to the vertical, is due 

 to the manner of growth, though frequently such positions 

 are much influenced by the conditions of the environment. 

 He spoke of these two positions as orthotropic and plagio- 

 tropic respectively. In 1882 Vochting showed that when 

 plants are withdrawn from all external influences their axes 

 of growth are nearly or quite straight, whether they are 

 orthotropic or plagiotropic. He said they possess a ten- 

 dency which he calls rectipetality. F. Darwin, writing in 

 1891, attributed this apparently straight growth to the 

 continual correction of slight curving departures from the 

 straight line. 



Though these fixed positions suggest that growth takes 

 place always in a straight line, this is seldom realized 

 even if conditions are kept constant. The line or direction 

 of the axis generally inclines first to one side and then to 

 the opposite, or else is directed to all points of the 

 compass in regular succession. It exhibits thus a new 

 rhythm, which is known as nutation in the first case 

 and circumnutation in the second. Sachs, in 1873, re- 

 stricted the term nutation to the movements of the 



