142 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



because in bamboo there is no evidence of the dispersion or low cellulose 

 content needed to make the explanation hold. 



It may therefore be concluded in general that the structure of these 

 elongated cells shows a crossed fibrillar organization somewhat like 

 that already found in some algae. We shall see in the next chapter that 

 the resemblance goes even further than this, so that it is salutary to note 

 here at least one major difference, a difference in the extent to which the 

 crossed fibrillar structure goes. In these elongated cells, the layers with 

 different directions are microscopically visible and therefore few in 

 number. In the algae, however, they are submicroscopic in thickness 

 and therefore many in number. Whether this is a really fundamental 

 difference or not it is impossible to say at the moment. We might 

 perhaps in passing note the different rates of development of the two 

 types of cell. In the algae each cell continues apparently to deposit 

 cellulose from the time it appears throughout the whole growing 

 season — and perhaps for more than one year in Valonia. With conifer 

 tracheids, a count of the cells developed over a week or two suggests 

 that a tracheid is completely developed within two days from its first 

 differentiation from the cambium. It is therefore a most interesting, if 

 dangerous, speculation that wall layering of this kind in the algae 

 has been associated with changes in the environment, where the 

 cell grows over a long succession of days and nights, while a conifer 

 tracheid grows only for about two days and one night (or two nights 

 and one day). 



Structure in other mature cell types 



While interest has centred largely around the elongated lignified 

 elements of plants deaU with thus far, observations have naturally been 

 made also on other cell types and we may perhaps glance at one or two 

 of these now. There is little that can be said of the majority of 

 parenchyma cells. The walls are commonly almost isotropic in face 

 view, and whether this means that the cellulose micelles are arranged 

 completely at random, or whether the wall is built up of submicroscopic 

 layers each with its own chain direction rather like minute Valonia cells, 

 is not known. In certain cases, however, the structure of parenchyma 

 cells has been worked out in some detail. The cells of coleoptiles in 

 particular have been the object of many investigations. The m.e.p. has 

 been stated to follow a rather flat spiral (50(«)) and to show a relation 

 to cell length very roughly as described later. This observation may, 

 however, need to be re-examined in view of more recent work in the 

 electron microscope (50(c)) which seems to show the presence in the wall 



