MECHANICAL ACTIONS 359 



of Vallisneria spirah's, but soon appears in the leaf-cells when a section is watched 

 under the microscope. It appears first near the point injured, and spreads for a short 

 distance through the parenchyma, but for a longer distance when the vascular bundle 

 is also affected. In such cases it may spread over the entire leaf, or even over all the 

 leaves on the plant *. The leaf of Elodea canadensis responds similarly, except that 

 streaming is often present in the intact leaf in the cells along the midrib, and here the 

 streaming may be so accelerated that the chloroplastids are drawn into it and circu- 

 late round the cells. Usually the effect of the stimulus gradually passes away and the 

 plant returns to its original condition. The same applies to the increased respiration 

 and production of heat which, together with streaming, are all signs of the increased 

 activity produced by an injury. Streaming is absent from certain cells under all con- 

 ditions and whatever stimuli be applied, while in other cases streaming begins without 

 any special external stimulus being required, and may then continue as in Chara and 

 Nitella for the whole life of the cell 2 . 



Frank observed that even in the absence of streaming an injury might cause 

 a marked change in the position of the chlorophyll bodies, and Tangl, Nestler, Ne'mec, 

 and Miehe 3 have shown that a cut or puncture causes, in a great variety of plants, 

 a more or less pronounced movement and collection of the protoplasm and nucleus 

 on the wall facing the injury. The time of reaction depends upon the plant and on 

 the external conditions, but in roots it may, according to Ne'mec, be shown in from 

 a quarter to several hours, and spreads with decreasing intensity from 0-5 to 0-7 mm., 

 according to Nestler, and even up to 1.3 mm., according to Ne'mec, from the point of 

 injury. 



After the maximum reaction has been reached in a few hours or a few days, the 

 aggregation is gradually redistributed. According to Ne'mec, this takes place so 

 rapidly in roots that the reaction has already ceased near to the injury by the time it 

 has reached its maximum in the furthermost zones affected. A secondary change, 

 consisting of the enlarging and fusion of the vacuoles, was then observed by Ne'mec, 

 but this spreads to a less distance than the primary reaction. Interesting as these 

 movements are, however, they simply form another indication of the wound reaction, 

 and do not give any insight into the causes of it. 



of preparation usually does not inhibit streaming or does so only temporarily. Kienitz-Gerloff s 

 discussions (Bot. Ztg., 1893, p. 36) show an ignorance of the nature of irritability, and the same 

 applies to I. Keller (1. c., p. 8), who considers streaming to be a purely pathological phenomenon. 



1 Hauptfleisch, 1. c., p. 196 ; Kretschmar, Jahrb. f. wiss. Bot., 1903, Bd. xxxix, p. 275 ; Ewart, 

 Protoplasmic Streaming in Plants, 1903, p. 104. [All three authors observed a more rapid propa- 

 gation longitudinally than transversely. Kretschmar observed a maximal rate of propagation in 

 Vallisneria of 3 cm. per minute, whereas the average rate of propagation observed by Ewart (I.e., p. 105) 

 in the leaf of Elodea was 1-3 mm. per minute at 30 C., and was barely more rapid in Vallisneria.'] 



2 [On the reasons for the absence of streaming in certain cells, cf. Ewart, 1. c., p. 29seq. In 

 small plant-cells diffusion from end to end is more rapid than streaming, whereas in large cells like 

 those of Chara and Nitella in which streaming is an essential factor in continued life the protoplasm 

 may stream several times round the cell during the time required for complete diffusion. Hence the 

 importance of streaming in large cells : cf. Ewart, On the Ascent of Sap in Trees, Phil. Trans, of the 

 Royal Society, 1905, p. 40.] 



3 Tangl, Sitzungsb. d. Wien. Akad., 1884, Bd. xc, Abth. i, p. 10; Nestler, Sitzungsb. d. Wien. 

 Akad., 1898, Bd. evil, Abth. i, p. 708 ; NSmec, Die Reizleitung u. d. reizleitenden Structuren, 1901, 

 p. 8 ; Miehe, Flora, 1901, p. 127. 



