I CARBOHYDRATES, CHITIN AND CUTIN 285 



tension growth in the classical sense of such a cell is not possible. 

 Growth in area is only realized in the two polar outgrowths, of which 

 only one is visible in Fig. 140a. The tip of such a process seen in 

 the electron microscope is shown in Fig. i4od. It is open, and evi- 

 dently the same weaving of a transverse weft into a longitudinal warp 

 takes place as was described above. 



RoELOFSEN (1951b) finds an axial orientation of the microfibrils on 

 the outer surface of the primary wall of cotton hairs and a tangential 

 orientation on the inner surface. He thinks that the outer fibrils have 

 been oriented by cell extension. It is more likely, however, that these 

 longitudinal microfibrils represent the "warp" as seen in Fig. 86b 

 (p. 128). 



The impossibility of wall extension is even better illustrated by the 

 epidermal cells (Figs. 140b, e). They elongate in the oat coleoptile 

 about 150 times, (Frey-Wyssling, 1945a), but during the whole time 

 of this rapid growth, which lasts four days, there is the compact outer 

 wall, several // thick, characteristic of the epidermal cells of plants. 

 The electron microscope discloses tip growth, not only for the thin- 

 walled interior part of the epidermal cell, but also for that very thick 

 exterior wall. It is an amazing thing that, simultaneously, in one and 

 the same cell, a tubular texture should be laid down for the interior 

 faces of the cell w-all and a parallel one for the thick exterior faces. 

 This fact argues against any simple physico-chemical origin of cell 

 wall textures comparable to that of molecular surface films. There are 

 unknown morphogenetical principles inherent in the cytoplasm 

 building the wall. From Figs. i4od and e it w^ould seem that the 

 cytoplasm oozes out of the cell in order to weave its wall, not only 

 from inside, but also from outside. 



The discovery of bipolar growth raises the question whether there 

 is any intercalation of microfibrils by intussusception. Hitherto the 

 growth in area was considered to consist in local expansions of the 

 wall and concomitant insertion of new cell wall substances into the 

 loosened area. The bipolar growth does not favour such a view, 

 because it consists essentially of an addition of a new area to the 

 existing wall and not in a general enlarging of the cell faces by internal 

 growth. However, there are growth phenomena, such as the enlarge- 

 ment of the cross-section of plant cells, which cannot occur by the 

 simple additon of new wall areas. This growth in area consists in 



