I CARBOHYDRATES, CHITIN AND CUTIN 283 



hyphae etc. On the other hand, very rapidly expanding cells in the 

 tissues of coleoptiles, hypocotyls, radicles, staminal jfilaments etc. were 

 thought to elongate by increasing their cell surface along its total 

 length owing to passive extension accompanied by active intus- 

 susception. 



The process of the addition of new microfibrils to the existing 

 texture in tip growth is difficult to observe in the electron microscope. 

 In growing root hairs the apex appears to be covered by a felt of 

 cellulose microfibrils which stiffen the slime around these cells (Frey- 

 Wyssling and Muhlethaler, 1949 b), and those of the pollen tubes 

 (VoGEL, 1950) or of sprouting sporangiophores (Frey-Wyssling and 

 Muhlethaler, 1950) are so intensely cutinized that the cellulose 

 texture is obscured. Cells which grow in water do not present these 

 difficulties. In the end cell of a Sp'iro^'ra thread the microfibrils are 

 not intermeshed (Fig. 86b, p. 128). The tip consists of loose longi- 

 tudinal microfibrils which represent a kind of warp. At their distal 

 end these microfibrils seem to be free, whilst at their base they are 

 tied together by transverse microfibrils which function as a weft. In 

 this way a woven texture results. Soon the number of transverse 

 microfibrils exceeds that of longitudinal fibrils, thus producing the 

 optical negative reaction of the fully grown primary wall. 



In order to investigate the so-called extension growth, elongating 

 coleoptiles were macerated and the isolated cells duly prepared and 

 observed in the polarizing and the electron microscope (Muhle- 

 thaler, 1950b). The result of this research is very surprising. It 

 transpired that there is no extension of the wall in its total length, 

 but the cell elongation is due to a rapid bipolar tip growth. This is 

 illustrated by Fig. 140. Picture a) shows an expanding parenchyma 

 cell of the oat coleoptile stained with benzoazurin in the polarizing 

 microscope. The dichroism of this dyestuff produces deep coloration 

 when the direction of the bulk of the microfibrils coincides with the 

 vibration plane of the polarizer. It is seen from Fig. 140a that there 

 is a heavily stained cell body with pits from which a long thin-walled 

 outgrowth protrudes. In the cell body longitudinal ribs are visible 

 which correspond to the cell edges. Fig. 140c gives a detail of such 

 a rib with the adjacent pitted primary wall in the electron microscope. 

 It is evident that a wall fortified by numerous parallel textured ribs 

 cannot be extended in the longitudinal direction. Therefore, an ex- 



