THE HIGHER CRYPTOGAMIA. 231 



open in the spring are completed in all their parts, and 

 after the second winter's rest they are fully developed. The 

 younger fronds of the same season follow step by step in 

 the same development until the month of June. 



The commmencement of the formation of the vascular 

 bundles takes place in the bud even of very vigorous speci- 

 mens from the fifth-youngest frond in a backward direc- 

 tion, and thus, far above the point at which the longi- 

 tudinal growth of the stem begins to exceed its growth in 

 thickness. Thus the whole system of vascular-bundle- 

 meshes lies at first in an almost horizontal, very flatly para- 

 boloidal surface, close under the top surface of the stem, 

 and nearly parallel thereto. It is only immediately below 

 the apex that the number of the cells of the tissue of the 

 stem underneath and within the net of vascular bundles is 

 increased ; lower down there occurs an expansion of these 

 internal cells, their longitudinal diameter becoming from 

 four to five times longer, and their transverse diameter 

 from two to three times wider. It is only by this increase 

 (caused by cell-expansion) of the bulk of the pith that the 

 net of vascular bundles is lifted up by degrees and pro- 

 jected upon a cylinder. It is easily seen by counting the 

 cells during and after the transition of the net of vascular 

 bundles from the form of a paraboloid to that of a cylinder, 

 that the increase in thickness of the stem is not caused 

 by any subsequent new formation of parenchymatal cells 

 either within the pith or in the neighbourhood of, or be- 

 tween, the rudimentary vascular bundles. It is only in 

 front of the youngest rudimentary vascular bundles that a 

 slight multiplication of the cortical tissue takes place, by di- 

 vision of the peripheral cells (PI. XXVII, fig. 3). 



If any radial section be taken through the longitudinal 

 axis of the stem the side view thus obtained of the apical 

 cell of the terminal bud is without exception three-sided (PL 

 XXVII, figs. 3, 4). When viewed from above the upper 

 surface of the same cell exhibits the like shape (PL XXVII, 

 figs. 1,2). Its form is therefore that of an inverted three- 

 sided pyramid with an arched upper surface. The appearance 

 shews (PL XXVII, figs. 1, 2,) that this cell divides re- 

 peatedly by septa, having three directions, and turned sue- 



