178 The Anatomy of Plants BOOK n 



in the region of the middle lamella of the cell wall, and he 

 interpreted this dot as the persistent nodal dot of the 

 achromatic fibrils. From this dot the thread appeared to 

 develop through the substance of the wall in both direc- 

 tions. 



It is interesting to note that similar swellings or dots 

 were observed by Russow in the middle of the threads or 

 perforating fibrils of the sieve tubes. 



Gardiner's view, that the perforations are coeval with 

 the formation of the wall and not produced as it grows 

 older, was borne out by his observations on the distribu- 

 tion of the threads in the walls of such oblong cells as exist 

 in the cortex of young growing stems. The end walls, which 

 undergo but little increase of surface due to stretching, 

 show groups of the threads nearly uniformly distributed 

 over them j on the side walls, however, they are more 

 or less grouped into areas separated by certain distances 

 from each other. Here the influence of stretching can 

 be seen, the separation of group from group being attributed 

 to such a force exerting its influence most strongly on such 

 areas as contain the least number of threads. 



The last contribution to the subject made in the century 

 was a very complete account of the structure and distribu- 

 tion of the threads in Pinus, made by Gardiner and Hill 

 in 1900. Though the most elaborate memoir of the time, 

 it only confirmed and extended the observations already 

 described. 



The study of the structure and behaviour of the nucleus 

 was the most engrossing feature of research on the cell 

 during nearly the whole of our period. When first dis- 

 covered by Robert Brown, its importance was not recog- 

 nized, and it only slowly assumed the preponderance which 

 marked its position after the early seventies. 



It was realized some years later, in 1884, that it is 

 concerned in the problems of inheritance, a result due to 



