386 The Physiology of Plants BOOK in 



separation of the layers, new ones being then intercalated 

 between them. 



In a paper published in 1881, the year following his 

 discovery of the action of the plastids, Schimper discussed 

 the question from the new point of view. He agreed with 

 Naegeli that the formation is accompanied by the setting 

 up of tensions, but he opposed the theory of intussuscep- 

 tion and of the intercalation of new layers between the 

 old ones. His explanation was that the tensions are fol- 

 lowed by a swelling of the starchy substance along lines 

 formed by the stretching which the tensions cause, the 

 so-called nucleus (kern) of the grain being the first swollen 

 part. The successive watery layers follow in sequence 

 owing to the resumption of the tension after the relaxa- 

 tion which follows the formation of the kern. He saw in 

 the formation consequently a succession of increased and 

 diminished tensions in consequence of repeated swelling. 



Strasburger was of opinion that the darker lines of the 

 striation do not indicate bands, but are only the separation 

 surfaces of the successive translucent layers. 



Correns, Binz, and Meyer brought forward again in 1891 

 and 1892 the view of Naegeli that the striations indicate 

 the alternation of layers of greater and less density. Acqua, 

 in 1894, said that the cell protoplasm invests both plastid 

 and starch grain and deposits layers of starch round both. 



The most careful researches that have been published 

 on the subject are those of A. Meyer in 1895 and Salter 

 in 1896, to which allusion has already been made. 



Meyer's observations were made on several plants, but 

 his chief results on the point under discussion were derived 

 from observations made on Adoxa and Pellionia. He said 

 that the grains are built up of individual laminae of rela- 

 tively lax and dense consistency, the former staining most 

 easily. The primary differentiation is induced by varia- 

 tions which affect the course of deposition and particularly 



