268 HOFMEISTER, ON 



(PL XXXV, fig. 3). The middle one of the three cells 

 which are produced in the interior of the cell of the second 

 degree (all of which lie in one plane) is the only one which 

 reaches to the middle point of the stem. It divides into 

 an inner and an outer cell by means of a longitudinal 

 septum almost parallel to the chord of the curved free outer 

 surface (PL XXXV, fig. 5). Both are divided by a trans- 

 verse septum, the outer one usually before the inner one 

 (PL XXXV, fig. 2). In the former this septum is parallel 

 to the upper and under surface of the cell ; in the latter it 

 is usually horizontal, and at right angles to the longitudinal 

 axis of the shoot (PL XXXV, fig. I). The stem forthwith 

 increases in thickness by repeated division of the cells of 

 the circumference by means of septa parallel to the free 

 outer wall. In very vigorous shoots, as for example in the 

 autumn shoots of Equisetum limosum, the like division occurs 

 several times in the cells of the next inner layer also 

 (PL XXXV, fig. 1). In such buds the cells of the circum- 

 ference usually divide once more by transverse septa close 

 above the place of origin of the youngest leaf. During the 

 increase of the stem in thickness the number of the cells of 

 its circumference increases continually by the division of 

 these cells by means of septa radial to the longitudinal axis 

 of the shoot or at least only slightly diverging from the 

 radial position. At first, in the upper part of the conical 

 cellular mass, this division alternates very regularly with 

 division by longitudinal septa parallel to the outer wall of 

 the cells ; lower down, where the stem becomes thicker, 

 division by a radial septum does not occur until after 

 several divisions by septa parallel to the axis of the stem. 

 The mass of the end of the stem which lies above the 

 youngest leaf, and the number of the cells of its longitu- 

 dinal axis and of its circumference are very different in 

 different species and even in the shoots of the same plant ; 

 they vary with the vigour of the shoots. By the early 

 occurrence of transverse division in the cells produced by 

 the multiplication of a cell of the second degree, the ladder- 

 like interweaving of the cells of the two longitudinal halves 

 of the stem is equalised at an early period. In those cases 

 in which this division does not extend into the cells adjoin- 



