THE FORM OF THE PLANT CELL 17 



wall apparently does not become thinner during growth, its solid sub- 

 stance does diminish per unit area even though the cell continues to 

 make new wall substance. It is always dangerous to argue from isolated 

 cases such as this, but the morphological similarities between growing 

 plant tissues make it seem possible that this may be a general phenome- 

 non. The new wall material which has been deposited must have been 

 laid down by one of two processes — either by plastering new layers 

 from within (the so-called apposition of von Mohl) or the insertion, 

 within the existing wall, of new particles of wall substance (the so- 

 called intussusception of Nageli). As the cell ceases to grow, however, 

 the wall certainly begins to thicken by the deposition of new layers from 

 within, and at about this time the cell begins to show the phenomenon 

 of plasmolysis. Now, therefore, that an interface appears to have de- 

 veloped between the cell wall and the protoplasm the new wall layers 

 may be thought of as deposited at a protoplasmic surface. The new 

 layers differ from the primary wall in several respects. They can often 

 be distinguished from it under the microscope in untreated transverse 

 sections and must therefore have different refractive indices, which alone 

 would indicate a difference in the submicroscopic structure, and they 

 often show different staining reactions. By now the cell, whether 

 elongated or not, contains a large vacuole which fills it almost 

 completely, and at this stage the cell wall is thought of as being 

 characterized by the presence of a thin lining of protoplasm on its 

 inner face, separating it from the vacuole. The secondary wall now 

 proceeds to develop to an extent which varies according to the type of 

 cell considered. In the isodiametric cells of parenchyma, the secondary 

 wall commonly remains thin even though the protoplasm remains alive 

 for some considerable period and retains its metabolic activity as 

 indicated by the storage within it of starch. In other cells, secondary 

 wall production proceeds until almost the whole cell volume is occupied 

 by the wall; this occurs in many phloem fibres for instance, where the 

 cell therefore sometimes looks like a solid rod with a narrow thread- 

 like cavity running down the centre. In the majority of elongated cells 

 — wood fibres and tracheids, collenchyma cells, many phloem fibres — 

 and in some which are not so elongated — vessels in dicotyledons, 

 vegetative cells in algae, etc. — the wall becomes appreciably thick but 

 the protoplasm either degenerates or ceases to deposit cellulose before 

 the cell becomes filled. In the elongated cells of the higher plants — 

 — e.g. wood fibres and tracheids — the protoplasm actually dies and the 

 cell contents disappear, leaving a hollow thread consisting of the 

 thickened wall envelope surrounding the lumen. It is with cells of this 



