390 LECTURE XVI. 



are exhibited by shoots which grow from rhizomes and 

 tubers, and which are therefore abundantly supplied with 

 plastic material. It might be suggested that the thinness 

 of the walls is due to the excessive growth in length of the 

 cells. This explanation may be of some value as regards 

 epidermal and collenchymatous cells (see p. 337), but it does 

 not seem to be applicable to sclerenchymatous cells, for 

 Rauwenhoff observed, as mentioned above, that the hard-bast 

 fibres of an etiolated interned e of Polygonum cuspidatum were 

 not excessively elongated, and yet he found that their walls 

 were thin. It appears that light has some direct-influence __ 

 upon the thickening of cell-walls, but it is notjossible at 

 present to explain the nature of this influence. 



It cannot be denied that the thinness of the walls of these 

 cells which normally have thick walls is favourable to the 

 elongation of the parenchymatous cells of an etiolated inter- 

 node, but the most important factor in their excessive elonga- 

 tion is their own more active growth quite apart from these 

 mechanical conditions. Kraus admits that the proper elonga- 

 tion of these cells is of importance, but he maintains that 

 their elongation is due, not to a true growth ojjtheir wallaihy 

 the addition of solid substance, but merely to the taking up of 

 water into. -them. In support of this view he states that 

 the pith and cortex of growing internodes increase in weight 

 in consequence of an increase of the water which their cells 

 contain rather than in consequence of an addition of solid 

 substance. This view is, however, incorrect. Karsten had 

 already pointed out that etiolated seedlings of Phaseolus 

 multiflorus contain as much cellulose as normal seedlings 

 of the same' age, and Godlewski has since found by com- 

 parative analyses of the hypocotyls of normal (grown in 

 light but not supplied with CO 2 ) and of etiolated seedlings 

 of Raphanus that the total dry weight of organic substance 

 is approximately the same in both. It is true that he finds 

 that the hypocotyls of the etiolated seedlings contain pro- 

 portionately more water than those of the normal seedlings, 

 but the fact remains that the amount of organic substance 

 built up into tissue is about the same in both cases. The 



