ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 413 



nitrogen obtained by the plants directly from the atmosphere. He 

 ascertained in the first place that plants contain nitrates only when they 

 absorb these salts from the soil through the roots. Nitrates were found 

 in all plants growing in the soil, though often only in the roots. When 

 present in the parenchyme he believes it to be stored up there as a food- 

 material. With regard to the total amount of nitrogen in the plant, his 

 conclusion is that the presence of vegetation promotes a process which 

 tends to the increase of the nitrogen contained originally in the soil and 

 in the seeds as sown in the ground. He does not believe that the 

 accumulation of nitrogen in the soil is brought about by the root-tubers 

 of the LeguminosEe, but rather by small Algae or other chlorophyllous 

 Cryptogams, which are always found in the soil. It depends on the 

 presence of cells containing albuminoids, the development of which must 

 be regarded as an independent process not connected with those which 

 take place in the soil. 



(3) Irritability. 



Physical Explanation of Irritation-curvatures.* — Dr. F. Noll 



contests the view of Wortmann f that geotropic and heliotropic curva- 

 tures are due to the accumulation of protoplasm on the convex in 

 contrast to the concave side, and supports the theory that they are caused 

 by greater growth of the membrane on the convex side. The observa- 

 tions were made chiefly on the stem of Hippuris and on the haulms 

 of grasses. 



The author asserts that it can be proved by experiment that there is 

 no difference in turgidity between the two sides of the cell, and that 

 the extensibility of the membrane is greater on the convex than on the 

 concave side ; the latter is proved by mechanical bending, the former by 

 the plasmolytic method. Measurements under the Microscope also 

 show, at the commencement of the curvature, a smaller thickness 

 of the membrane on the convex than on the concave side ; but this is 

 afterwards neutralized by the apposition of new layers on the thinner 

 wall. 



The physical process in irritation-curvatures consists in the mem- 

 brane (in unicellular organs or non-cellular plants) or membranes (in 

 multicellular plants) of the side which becomes convex becoming more 

 capable of extension, and therefore growing more rapidly in length, 

 than that of the concave side. The greater extensibility, or decrease in 

 elasticity, of the membrane on one side is due to the activity of the 

 parietal layer of protoplasm, in which the movable granular layer 

 takes no part. The parietal layer is excited to this greater activity by 

 external influences such as gravitation and light ; and it is this phe- 

 nomenon which is known as " irritation." 



To this Herr J. Wortmann replies,J regarding it as very improbable 

 that the difference observed by Noll in the thickness of the membrane 

 on the concave and convex sides of negatively geotropic organs is due 

 entirely to internal causes. External purely mechanical forces must 

 also take part in the phenomenon, and it is doubtful what set of causes 

 has the greatest effect. 



* Arbeit. Bot. Inst. Wurzburs, iii. (1888) pp. 49G-533 (4 figs.). 



t Cf. this Journal, 1888, p. 259. 



X Ber. Deutsch. Bot. Ge.ell., vi. (1888) pp. 435-8. 



