6 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



time, the security which comes from scrupulous care in observation and 

 a revulsion against unwarranted generalizations and abstractions. He 

 was the first to use the polarizing microscope to any purpose in this 

 study and, though he cannot be said in any way to have anticipated 

 Ndgeli, his interpretation of striations (fine markings seen on some 

 walls) in terms of lines of cleavage in crystals reveals the lines upon 

 which he was thinking. He considered the secondary wall to be de- 

 posited as a series of lamellae laid one upon the other by a process 

 which he therefore called apposition, a notion which fitted in so 

 admirably with the conclusions of Payen (1844) that young cell walls 

 consisted of rather pure cellulose, incrusting substance being added 

 later, that together they were able to contest successfully the curious 

 notion of Mulder and Hurting that the innermost layer of a cell wall 

 was the oldest. 



By the year 1850 von Mohl had come to regard the intercellular 

 substance (the modern middle lamella) as only a cementing material, 

 and other writers {e.g. linger) agreed. Some, however, including 

 Wigand and even Sanio, regarded it as the primary wall modified 

 chemically, and thus set in motion the confusion of terms which has 

 lasted until well into the twentieth century. 



Among many other distinctions of von Mohl, we may perhaps note 

 only that he gave the name of protoplasm (1846), first applied by 

 Purkinje (1840) to the formative substance of animal eggs, to the living 

 substance inside the cell. 



The general impression at this time was therefore of growing cells 

 with thin expanding cell walls of cellulose upon which lamellae were 

 plastered secondarily by apposition. The cells themselves were probably 

 cemented together with a formless cement, but in none of these 

 structures was any attempt made at an understanding of submicro- 

 scopic features or therefore at an impression of the processes of growth 

 which such knowledge alone can give. This was, however, soon to be 

 remedied in the capable hands of Ndgeli. 



The modern era 



Although Ndgeli was a contemporary of von Mohl, the outstanding 

 success of his application of physical principles to wall problems must 

 single him out as the first of the modern workers. The publication of 

 his Stdrkekorner in 1858 marked the beginning of a new era in wall 

 studies — and in the field of colloids generally — and of a tradition in 

 structural investigation at Zurich, where he worked, kept up so ably 

 in more recent times by Frey-Wyssling. For the first time, Ndgeli 



