10 



ANATOMY AND PHYSIOLOGY OF 



appears to occur in the strength of the expansion in the different 

 directions; but in the elongated cells of the bast and wood, the 

 swelling up resulting from moistening takes place principally m 

 the direction of the breadth, and only in a very small degree m 

 the longitudinal direction. 



The cell-membrane of young cells is completely colourless and 

 transparent ; in fall-grown cells it is frequently imbued with yel- 

 low, red, or brown colouring matters, whereby in many cases the 

 transparency is importantly interfered with. This alteration is 

 very striking in the change of the sap-wood into heart- wood, for 

 in many trees, e. g., in the ebony and yew, the white is converted 

 into a more or less dark colour, without the cell-membrane m- 

 creasing in thickness, while at the same time it acquires a far more 

 considerable solidity and independence of the influence of mois- 

 ture. 



Ohserv. It is difficult to conceive how some phytotomists (Link, ''Ele- 

 ment. Phil BoC 1824, p. 366 ; Meyen, ''PhysioU i. 30) came to the opinion 

 that cells contract in the direction of their length when moistened, and 

 again expand when dried, since, on the contrary, all cells expand in every 

 direction when moistened. In the elongated cells of the wood the con- 

 traction by drying in the direction of the length is, of course, but small, 

 yet it occurs constantly. In wood of Dicotyledons the longitudinal con- 

 traction from the wet to the perfectly air-dried condition amounts to only 

 0-072 to 0-4 per cent., while the contraction in tlie direction of the breadth 

 is as much as 4 to 9 per cent. According to Schleiden's experiments, the 

 bast-cells of flax expand only about 0-0005 to 0-0006 ; hut he considers 

 it possible that there was an important error here {Beitrdge, i. 69). Ac- 

 cording to the researches of Ernest Meyer, the Manilla hemp (Fhor- 

 mium ?J expands, when wetted, about l-50th of its length, whHe the 

 increase of breadth amounts to l-5th. 



I\ Structure. 

 In examining a transverse section of a thick walled cell^ e^g., 



Fig. 11. 







.\^ 





Transverse section throuu;'li 

 the liber-celis of Cocos bo- 



trijophora. 



cavity of the cell. 



Fig. 12. 



of wood-cells of Cle- 

 matis Vitalba, the 

 bast -cells of Palms, 

 (fig. II), or the thick 

 walled pith -cells of 

 Hoya carnosa, {^g. 

 12) we find by strong- 

 ly magnifying, that 

 the cell-membrane is 

 not homogeneous, but 

 composed of numer- 

 ous super -incumbent 



Transverse section through a thick lavCrS COUCeutrical- 

 walled cell of the pith of Hoi/a ^ *^ -,. ,-, 



carnom. \j surrouudiug the 



By tlie action of a mineral acid of proper 



^V'">'in 





