INSERTION OF PARTITION WALLS. 



96 



existing and dividing, new partition waUs arise, somewhat as if the chambers 

 of a house were each divided into two smaller rooms, by putting in partition 

 walls. The formation of cells in growing organs where the existing cells in- 

 crease in size, gives therefore the impression of each cell being only able to 

 attain a definite size; and, when it passes beyond this, a division wall appears 

 by means of which the cavity becomes divided generaUy into two equal halves. 

 In most cases these new partition waUs can be recognised in the framework of the 

 older cell-walls on transverse and longitudinal sections as thin lines, which are 

 usually set at right angles on the older waUs, as the accompanying figure clearly 

 shows. The cells akeady present grow; and as they grow and the chambers 

 formed by them become larger, partition walls arise, by means of which the 

 increasing size of the cells is to a certain extent again equalised, so that in a 

 given plant, in spite of the progressive growth, the cell cavities never exceed 

 a certain dimension. The man- 

 ner in which the new partition 

 walls are inserted in the growing 

 cells, gives in a certain sense the 

 impression of the latter being, so 

 to speak, cut through, and so 

 divided each into two portions. 

 The cell-division appears there- 

 fore (and this should be especially 

 insisted upon) as a diminution in 

 size of the chambers into which the 

 living plant substance is divided, 

 follovnng upon growth and con- 

 ditioned by growth. This mutual 

 relation of growth and cell -division 

 is particularly clear if one examines 

 from this point of view plants of 

 simple construction, such as the 

 Characece. Fig. 95 represents a 

 longitudinal section, through the 



apex or terminal bud of such a plant, which is preeminently suited, on careful 

 consideration, to render clear the relation of cell-division to growth and the whole 

 external and internal configuration of a plant. From the so-called apical cell (») 

 found at the end of the stem, each time this attains a certain height by gi-owth, 

 a lower transverse disk is cut off; and thus a new cell, a so-called segment, 

 is formed. This is again divided by means of a transverse wall into two cells 

 lying one above the other; a lower lenticular one, and an upper one which 

 resembles a biconcave lens. The former is no more divided on further growth ; and 

 gradually assumes the forms i" , i" , i"", by which, as seen, it becomes transformed 

 into a cylinder, continually increasing in length, and constitutes a joint or interfoliar 

 part of the axis of the Chara. The upper biconcave daughter-cell of the segment 

 developes, on the other hand, a number of outgrowths around the axis of the stem, 

 from which the leaves originate ; while the middle portion becomes the node between 



FIG. 94.— EpUlermis and .adjacent cortical parenchyma of the hypocotyl 

 of Heliattzktts annitus, which rapidly increases in thickness at the conclusion 

 of germination ; thedarkerthicker cell walls are the original ones, the thinner 

 radial ones those recently developed. The vigforous tang:ential growth of the 

 epidermis cells and the cuticle is especially interesting. 



