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from the fact that the stems of the former ones are exposed to be 

 broken by lateral pressures for instance by the wind, and therefore 

 it is most fortunate that the vascular bundles accompanied by bast 

 are arranged at the perifer of the stem by which resistance against 

 the lateral pressure grows the greatest. The aquatic plants, on the 

 contrary, are especially exposed to a pull in the length caused by 



Fig. 1. Small portions of the transverse sections of the stem of: A, P. lucens; 

 B, P. natans. — C and D, P. mucronatus (not P. pusillus as said in my book, 

 1. c. p. 44); C, transverse section of the stem (ca. 60: 1); D, a small portion of 



C, more enlarged. 



h, epidermis; sh, a stratum of cells between the epidermis and the outmost 

 lacunae; lg, lacunae; d, walls between the lacunae; g, chloroplasts ; s, bast; 



cs, axial cylinder. 



the current of the water, and in this case it is best that the 

 mechanical tissue should be gathered in one place and then espe- 

 cially in the middle of the stem. In the Potamogeton stem, there- 

 fore, we have besides the epidermis two sharply separated parties 

 viz. the axial cylinder and the bark outside; the barks inmost 

 peculiarly developed stratum of cells forms an endodermis round 

 the axial cylinder (figg. 1,0, 8; 9). 



