102 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



data on the growing wall of fibrous cells is at hand. It will be seen then 

 that, if the question is asked in the right way, there is some connection 

 between growth and wall structure. 



It is from this point of view that the filamentous algae are of especial 

 interest. As far as shape goes they form a bridge between forms like 

 Valonia and the cells of the higher plant, and they occur in such a 

 variety of species that it is an exciting business to see whether there is 

 any common underlying factor in the wall corresponding to the fila- 

 mentous habit. The results of such determinations as have been made 

 up to date cannot in any sense be regarded as final, but so far as can be 

 seen at the moment the filamentous algae fall into three main groups. 

 Only two of these will be discussed here; consideration of the third must 

 be postponed until the X-ray diagram can be interpreted! Attention 

 may therefore best be given first to those forms which most resemble 

 Valonia. 



Group 1. Cladophora, Chaetomorplia, Rhizoclonium, etc. 



These forms are natives of the temperate zone and therefore occur, 

 in greater or less abundance, around the coast of Britain as well as in its 

 fresh waters. Cladophora is typified as a much branched, multicellular 

 filament (Fig, 38), each cell containing many nuclei. One species, CI. 

 prolifera, is much larger than the others, and this was the first to be 

 studied. In all species the branches occur as a bulging of a cell at the end 

 nearer the filament tip (Fig. 36(a)) giving the appearance of being 

 "blown" out from the parent cell. Growth occurs exclusively in the 

 upper part of the apical cell, causing cell elongation which is followed 

 regularly by transverse division into two cells. The sub-terminal cell is 

 capable of limited growth in that the half of it nearer the filament tip 

 continues to extend for some little time after it has been cut off from the 

 terminal cell. The branch cells grow in a similar way through the agency 

 of a tip cell, so that the form of the plant becomes somewhat compli- 

 cated. In one species, CI. gracilis, the branches along any one filament 

 occur first on one side, then on the other, with some regularity. The 

 whole plant may be up to three or four inches in length. The organiza- 

 tion of the other two genera, Chaetomorpha and Rhizoclonium, is some- 

 what similar, except that the filaments are usually unbranched and 

 growth is not confined to the tip cells. 



With one exception, it is impossible in these species therefore to 

 investigate single cells in an ordinary X-ray spectrometer, and bundles of 

 parallel cells must be used. This introduces difficulties in the case of 

 Cladophora but even here, with care, it is possible to arrange a sufficient 



