352 HOFMEISTER, ON 



The above-mentioned two-fold longitudinal division of the 

 string of elongated cells which become differentiated in the 

 interior of the leaf and root does not occur until after the lat- 

 ter have emerged from the prothallium. The divisions appear 

 to take place contemporaneously throughout the entire 

 length of both the cellular strings which unite underneath 

 the terminal bud. The division by transverse septa of the 

 basal cells of the leaf, continues even after the commence- 

 ment of the formation of the vascular bundle, although in 

 a less degree; and the cells of the string which goes to 

 form the vascular bundles take part from time to time in 

 the division. One single longitudinal row of cells is en- 

 •tirely exempt from it. This row originates in the middle 

 of the fore side of the vascular bundle, by supplementary 

 longitudinal division of a row of cambial cells. In the 

 cells of this row, whose length — owing to the entire sup- 

 pression of all transverse division — far exceeds that of all 

 the neighbouring cells, thickened annular threads soon 

 make their appearance, passing here and there into spiral 

 threads (PL XLVII, fig. 3). The continual longitudinal 

 growth of the surrounding tissue stretches and distorts the 

 young vessel, and removes the annular threads to a consi- 

 derable distance from one another. 



In like manner there appears on the inner side of the 

 rudimentary vascular bundle of the root a row of elongated 

 spiral and annular cells of a prosenchymatal form like the 

 vessels of the leaf. In the first node of the plants, at the 

 place where the precursors* of the vascular bundles of the 

 leaf and root unite underneath the rudiment of the second 

 leaf, more than one of the longitudinal rows of cambial 

 cells assume a prosenchymatal form, and thickenings of the 

 walls are formed in all of them (PI. XLVIII, fig. 1). 

 These cells, which are the first rudiments of the wood, are 

 short and spindle-shaped, t and already bear some resem- 

 blance in form to the cells of which the principal mass of 

 the wood of the mature plant will consist. 



Many of the cells of the tissue which adjoins the vascular 



* Precurseurs Mirbel. 



f The intercalary transverse division has not extended to the cells adjoining 

 these. 



