IRRITABILITY 199 



that the direction of the divisions of the fertilized egg-cell 

 and of the first cells in the embryo bears a definite relation 

 to the force of gravitation, and this appears in many 

 plants to be really the case, but other influences may co- 

 operate or predominate in producing the same effect, as, 

 for example, in the ferns.* Whatever may be the origin 

 in the embryo of the different responses which the differ- 

 ent parts make, it is evident that, from the germination 

 of the seed onward, the plant is sensitive to gravitation 

 and is directed in its growth by it as well as by other 

 forces. 



The responses to the force of gravity are much better 

 known than are the immediate effects of the force in the 

 sensitive parts. Most of our knowledge and interest in the 

 subject are due to Ciesielski,t Darwin,! Sachs, and Pfeffer 

 and their followers. 



The young root is more highly differentiated, anatomi- 

 cally and physiologically, than its simple external appear- 

 ance suggests. At the extreme tip is the cap, a protective 

 covering of the "growing point." The "growing point" is 

 a mass of permanent meristem which gives rise to all the 

 root structures. Behind this is the region of evident growth, 

 where the young cells are increasing in volume by the ab- 

 sorption of water from their older neighbors (see fig. 9, 

 p. 168). Still further back and adjoining this region is the 

 zone where, through the root-hairs, water is principally 

 absorbed from the soil. If a young seedling with a straight 

 root be laid so that its root will be horizontal, care being 

 taken that the root remain moist, the tip will begin to 



* See Goebel, K. Organographieder Pflanzen, pp. 188-f, 1898. Campbell, 

 D. H. Mosses and Ferns. 1895. McMillan. C. The orientation of the 

 plant-egg and its ecological significance. Botanical Gazette, vol. 25. 1898. 



f Ciesielski. T. Untersuchungen iiber die Abwartskrummiing der Wurzel. 

 Beitrage z. Biologic d. Pflanzen. Bd. I., 1872. 



\ Darwin. C. The Power of Movement in Plants. 1880. 



Sachs, J. von. Different papers from 1873 to 1879, collected in his 

 Gesa melte Abhandlungen iiber Pflanzenphysiologie. Bd. II., 1893, and in 

 his Lectures on the Physiology of Plants. Eng. transl.. 1887. 



Pfeffer, W. Geotropic sensitiveness of the root-tip. Annals of Bcftany, 

 vol. VIII., 1894. 



