7^5 



pronounced; this confirms the investigations of Wieler and Hartleb^ Such 

 an effect can occur without an indication of a disturbance in growth by any 

 striking external characteristics. As a rule, however, this disturbance in 

 growth is accompanied by a discoloration of the chloroplasts and their subse- 

 quent balling. There then follows a contraction of the primordial sack and 

 a shrivelling of the chlorophyll grains. The leaf thus injured may still at 

 times live out its life normally, depending upon the intensity and length of 

 action of the chlorin. Usually, however, it dies prematurely, in part or 

 entirely. In the latter case, principally the leaf parts die, for which, because 

 of their position and the lesser development of mesophyll and vascular 

 bundles, the supply of water is acquired less easily and is smaller; these are 

 the tips and edges of the leaf. Therefore, we find dry, discolored leaf tips 

 in grain and narrow dry outhnes on both sides of the lower part of the leaf 

 surface which still remains green. As a result of rapid death, a compara- 

 tively important condition is found in the cell content of the dead parts. 

 The drying with the retention of air in the tissue is connected with a shriv- 

 elling of the cells ; yet in such a way that the walls of each cell do not touch 

 one another. The natural process of dr^dng, on the other hand, which 

 occurs only after complete impoverishment of the cell content, is character- 

 ized by the entire collapse of the mesophyll cells, in which the upper wall 

 falls against the lower wall and the whole flesh of the leaf, formerly green, 

 represents a pale straw colored strip of dense tissue with curving walls lying 

 upon one another in layers. The collapse of the cells in dift"erent varieties 

 of grain, with the exception of barley, extends almost entirely in the meso- 

 phyll during the natural process of dr}'ing, while the epidermal cells retain 

 approximately their normal height. In barley (characterized by practical 

 workers as "soft"), the epidermal cells also collapse in a natural death. But 

 in this, some of the widest cells of the upper surface form an outward fold. 

 In a cross-section through the dead leaf this appears as a conical protuber- 

 ance resembling a hair and gives the whole cross-section the appearance of a 

 thin, knotty spiny cord. 



Because of the importance of distinguishing a leaf which has died a 

 natural death from one destroyed prematurely by acid gases, we will illus- 

 trate a leaf injured by acids and one which has died normally. Fig 162 / 

 is the cross-section through the edge of an oat leaf dried by hydrochloric 

 acid, or chlorin vapor. It is seen that the tissue has shrivelled greatly, 

 especially between the ribs (the intercostal fields) zvithout the mesophyll 

 having had time to become empty. The cell contents appear a dirty green 

 to a brownish green color and variously contracted. The walls of the bast 

 layers at the angles of the leaf (B) and below the vascular bundles (b) like 

 the epidermis are colored a reddish yellow to a brownisJi yellow and the 

 epidermal cells in places {s) are so dried that the upper wall touches the 

 lower wall. Fig. 162, .? is a magnified cell group from 162, /, showing the 

 still abunda nt cell content. 



1 Wieler, A., and Hartlel), R. tjber Einwirkung- dei- Salzsiiure auf die Assimi- 

 lation der Pflanzen. Ber. d. Deutsch. Bot. Ges. 1900, p. 348. 



