3^ PLANT GROWTH SUBSTANCES 



exist. (Fig. i) Miihlethaler's excellent micrographs demonstrate that the 

 primary membrane is composed of a network of microfibrils in contrast 

 to the more or less parallel arrangement of the fibrillar structure in the 

 secondary wall. The sharply defined microfibrils of the primary mem- 

 brane are fairly constant in diameter (200-250 A) and seem to be partly 

 interwoven as in a textile fabric. 



Inasmuch as the fibrils of the secondary wall are arranged parallel to 

 each other, fine details of structure can be studied under the ordinary 

 microscope. On the other hand, since the microfibrils of the primary 

 wall are in the form of a network and the fundamental units of the 

 network are below the limits of microscopic visibility, details of primary 

 wall structure under the ordinary microscope cannot be worked out in 

 an entirely satisfactory manner. However, it has been known from studies 

 using crossed nicols, that the microfibrils of the primary wall are not 

 randomly arranged even though they are in the form of a network. It can 

 be assumed that the majority of fibrils composing the network are 

 oriented more or less in one direction. It has been known for years that 

 the cellulose chains in parenchyma cells are oriented chiefly perpen- 

 dicularly to the major axis of the cell and that this orientation does not 

 change during growth. Striking preparations showing the orientation 

 of the cellulose in the primary wall may be seen when cells are stained 

 deeply in congo red and viewed under a polarizing microscope (Fig. 2). 

 It is well known that congo red accentuates the weak birefringence of 

 cellulose. The stain is prepared as a 0.2 per cent solution of congo red in 

 I per cent solution of sodium hydroxide. Pieces of tissue (e.g. whole 

 Avena coleoptiles) placed in the dye solution for 48 hours may be 

 macerated easily on a slide by a tap on the coverslip. Thus it is possible 

 to study the orientation of the cellulose in single cells, and even single 

 walls are also easily obtained by allowing a slide prepared in the above 

 manner to dry with a weight on the coverslip. The upper walls of many 

 cells adhere to the coversfip while the lower walls adhere to the slide. 



Some years ago, a large number of various primary wall types were 

 studied under the polarizing microscope and from these studies certain 

 generalizations may be made. Whenever a cell possesses an elongated 

 shape, the main orientation of the cellulose fibrils and the major orienta- 

 tion of the primary pit fields are perpendicular to the major cell axis. 

 The regions of the wall in contact with the intercellular spaces, show a 

 second orientation, for here additional fibrils are oriented parallel to 



