655 



light is shut away. Further, it will develop so much the less, the fewer the 

 cells originally formed as leaf primordia at the tip of the stem; a clasping 

 leaf, on this account, will develop further than a whirl leaf can, because, 

 in the primordia of the former, the whole circumference of the stem is 

 active, while in those of the latter, the cells at the same height on the stem 

 must be divided among as many leaves as the whirl numbers. A further 

 point, which must be of influence on the development of the leaf in the 

 dark, is the distance of the leaf primordia from the soitrce of the reserve 

 substances. Those produced first, and lying nearest a reserve substance 

 store, remove more material from the supply and, on this account, become 

 larger than those produced later and higher up on the etiolated stem. Thus 

 the development of the etiolated leaf is dependent on the individual 

 primordia and on the amount of nutrition to be found in its immediate 

 proximity. 



The primordia of the monocotyledon leaves, in the majority of cases, 

 are formed like a roll, surrounding the stem, below the vegetative cone and 

 in the immediate proximity to reserve substance stores, when these are 

 present, from which the- dissolved constructive material has to pass only a 

 short distance through the shortened axis (grasses). 



Having discussed the etiolation phenomena of the leaf, the unusual 

 elongation of the etiolated stem members remains to be explained. We will 

 follow in this the statement made by Kraus^. As a rule, etiolated stems are 

 thinner than normal ones, caused by a lesser number of cells, and this 

 deficient activity in the cambium of the stem is explained by the assumption 

 that some of the nutritive substances, worked up by the leaf, which pass 

 over into the stem through the petiole, pass further in a radial direction and 

 help to nourish the cambium of the internode of the stem. If this source of 

 nutrition fails, i. e., the leaf, which in the dark remains in the form of a 

 scale, is not in a position to obtain material for cell increase, the stem mem- 

 ber remains as it is without any actually new cell formation. The thicken- 

 ing of the cell walls is also suppressed. In normal stems the parenchyma 

 cells of the bark and the prosenchyma cells of the wood become thickened 

 during their growth in length. The pith cells, however, begin to grow 

 thicker only when elongation is approximately at an end, i. e., at the latest 

 moment, since they are only reached by the cellulose micella, wandering in 

 a radial direction from the leaf into the interior of the stem, when it is no 

 longer used to thicken the wood or bark cells. In etiolated stems, because 

 of the lack of nutrition, the thickening of the cell is only indicated, so that 

 it is almost lost in those which lie between the different vascular bundles 

 and, in the normal condition, develop into wood cells. On this account, 

 frequently no closed wood ring is found in the etiolated plants. The loss 

 in thickness suffered by these cells is compensated for by their greater 

 length, which exceeds that of the normal cell from two to four times. 



1 Kraus, C, Tiber die Ursachen d. Formveranderungen etiolierender Pflanzen. 

 Prlngsheim's Jahrb. f. wiss. Bot., Volt VII, Part 1, 2, p. 209 ft. 



