29 



the sclerenchyma and collencliyma may even be entirely sup- 

 pressied. , 



In most cases, hypoplasia is caused by disturbances in 

 nutrition,- it boing immaterial whether shade plants and 

 shade leaves are concerned, plants cultivated under watei 

 or in places saturated with vs.por, v;hose transpiration 

 current is thus insufficient for providing them with the 

 necessary food stuffs, also etiolated specimens, or even 

 when the disturbance is caused by infection with parasitic 

 ftogi.l In all cases, as. far as the mombranoe is concerned 

 the same symptoms are involved. At the same time, in all 

 plants having insufficient transpiration currents, we may 

 prove that there is only a weak development of the cuticle 

 of the epidermal cells. Weakly developed cell-membranes 

 are frequently found in dwarfed specimens, in which the for- 

 mation of the mechanically effective tissue is often en- 

 tirely suppressed. 2 when diatoms under unfavorable life 

 conditions show a more weakly developed shell structure 

 than under normal conditions, it may be traced back to sim- 

 ilar arrestment of the growth in thickness. According to 

 34 Heribaud ^, the striation of the shells of Gomphoema, Uav- 

 icula, Stauroneis, and Synedra, is only slightly pronounced 

 when the cultures are kept in weak light. According to 

 Karsten ^, the formation of the little silicious rods in 

 Sceletonema costatum is suppressed when the organisms are 

 left absolutely undisturbed on the bottom of the culture 

 dish. In the first case, the arrestment of the cell-mem- 

 brane development may be traced back to the decreased as- 

 similatory activity of the cells; in Karsten' s experiment 

 the dormant cells will also have been placed under more un- 

 favorable nutritive and respiratory conditions than those 

 cells which, in uninterrupted passive motion, are carried 

 continuously to layers of v/ater containing food-stuffs, 

 and rich in o^gen. 

 , jt 



1. Compare the above mentioned literature on shade 

 leaves, as well as the articles quoted in Chapter IV, 2. 

 Further, Wakker, Untersuchungen fiber den Emnfluss parasit- 

 ischer Pilze auf ihre Hahr-pfl. , Pringsheim' s Jahrb. f. v\riss 

 Bot., 1892, Bd. XXIV, p. 499. Tubeuf, Pflanzenkrankh. 

 durch kryptogame Parasiten verursacht, Berlin 1895, p. 53, 

 ff. Kny, Eine Abnormitat in der Abgrenzung der Jahresringe, 

 Sitz.-Ber, Katurg.- Pr, Berlin, 1890, p. 138 (Thin walled 

 autumn wood) . It is very remarkable that in badly nour- - 

 ished specimens the cells in the central cylinder of the 

 roots have abnormally thick ?;alls . (Observations on water 

 cultures by Pethybridge loc. cit.) . (Compare Chapter III), 



2» Compare especially Gauchery, loc. cit. 



3« De I'infl. de la lumiere et de 1' altitude sur la 

 striation des valves des Diatomees. C. P.. Acad. Sc. Paris, 

 1894, T. CXVIII, p. 82. 



4, Die Pormveranderung v. Sceletonema costatum (Grev) 

 Grun und ihre Abhangigkeit von ausseren Paktoren. Wissensch. 

 Me ere sunt er such. , 1898, Bd. Ill, p. 13. 



