772 SUMMARY OF CURRENT RESEARCHES RELATING TO 



In roots entirely submerged in water, there is a well-developed 

 intercellular system, while the vascular system is less developed. 

 When an aquatic plant is transported on to dry land, the intercellular 

 system diminishes, while the vessels become more numerous, and 

 lignifieation is carried on further. 



Growth of Roots.* — E. v. Wettstein thus states the laws which 

 govern the growth of roots : — 



1. In the first periods of development the growth is uniform; 

 afterwards, from the period of germination, it is localized ; the position 

 of the zone of maximum growth varying. It begins at the collar, 

 advancing gradually towards the apex, where it ceases. 



2. The nearer the growing region approaches the apex of the root, 

 the less rapidly does it advance. 



3. The length of the growing region increases as it approaches the 

 apex of the root, attains a maximum, and then decreases. 



4. Neither the nature of the environment nor variations in tem- 

 perature exercise any influence on the law of growth ; even decapitation 

 may not essentially alter the course of growth, at least at first. 



5. As long as the region of most vigorous growth has not ap- 

 proached within about 4 mm. of the apex, the growth of the young Toot 

 depends only on the elongation of the cells already formed in the seed. 

 The first stage of growth is the result of this elongation taking place 

 in fresh layers of cells, and the growing region thus advancing towards 

 the apex. 



6. When the zone of maximum growth has advanced to within 

 4 mm., or less, of the apex, cell-division and elongation of cells 

 go on pari passu. In the second stage of growth the cells freshly 

 formed near the apex contribute to the growth of the root by their 

 elongation. 



7. The first stage of the growth of roots is independent of the 

 conduction of reserve-materials from the cotyledons or endosperm. 



8. " Sachs's curvature " depends on a difference in the growth of 

 the two sides of the root. This fact is in harmony with Wiesner's 

 explanation of the occurrence of spontaneous phenomena of nutation 

 in other organs. 



Growth in length of decapitated and uninjured Roots.t — 

 H. Molisch confirms Wiesner's statement that roots when deprived 

 of their growing point grow less in length than uninjured roots under 

 similar conditions of growth ; and that this difference of growth in 

 length depends greatly on temperature, being inconsiderable when 

 the temperature is low. He further believes that the reasons why 

 Kirchner has come to a different conclusion are probably that he 

 worked at too low temperatures ; that he removed too small a 

 quantity from the apex of the root ; and that the number of ex- 

 periments performed was not large enough to arrive at definite 

 conclusions. 



* Anzeig. K. Akad. Wiss. Wien, Feb. 14, 1884. See Bot. Centralbl., xvii. 

 (1884) p. 359. 



t Ber. Deutsch. Bot. Gesell., i. (1883) pp. 362-6. 



