differences in the Structure of Stems. 23? 



tain only comparisons between the so-called woody trunks of 

 palms, and those of the dicotyledonous forest trees of our 

 zone, the consideration being for the most part overlooked, 

 that things wholly dissimilar, and which will not admit of 

 comparison, are thus compared. The palm-stems, for ex- 

 ample, originate from the undeveloped interfoliar parts, but 

 our dicotyledonous woody stems from the developed parts; 

 and this distinction is so very essential for plants with nu- 

 merous series of woody bundles, that the stalk of a pink, and 

 a culm of grass, do not differ so much as the latter and the 

 stalk of a bulb. It appears to me, that even if the most cor- 

 rect, certainly not the shortest expression, has yet been found 

 for the distinction between the two great divisions of the 

 Phanerogamia, with reference to the structure of their stems, 

 notwithstanding all the researches of the most eminent na- 

 turalists. In stems in general the following differences occur, 

 which rest on the development, number and arrangement, 

 direction and structure of the vascular (woody) bundles. 



1. The vascular bundles, whose development always proceeds 

 from the interior to the exterior, are either limited or unlimited 

 in their growth. Commonly every vascular bundle consists 

 of three different physiological parts, namely of an extremely 

 delicate, rapidly developing tissue, of most tender texture, in 

 which new cells are continually generated ; these are depo- 

 sited in various configurations, in two different directions, viz. 

 towards the exterior, in the shape of a peculiar, very thick- 

 walled cellular, more or less elongated tissue {liber) ; and to 

 the interior, in gradual succession (subjected to the gradual 

 expansion in length of the part), in the form of annular, spi- 

 ral, reticulate, and porous vessels ; and of ligneous cells, the 

 latter either uniform, or differing from each other, forming the 

 wood, properly so called. Up to a certain period the deve- 

 lopment of the vascular system in the Monocotyledons and 

 Dicotyledons proceeds uniformly ; but then, in Monocoty- 

 ledons the active formative cellular tissue, with delicate 

 walls, suddenly changes ; the partitions of the cells become 

 thicker, their generating power ceases, and, when all the 

 surrounding cells are fully developed, they also assume a 

 form entirely peculiar, and cease to convey gum, mu- 



