PHANEROGAMIA : AXIAL ORGANS. 225 



however are still wholly pervaded by sap (second stage) ; lastly, the 

 cellular tissue, which has already attained a greater age, in the black- 

 ish appearance which is produced by the intercellular passages being 

 freed from sap, and containing merely air (third stage). When these 

 points are discriminated, the origin of the forms may be traced pretty 

 easily in most axes. 



I. The arrangement of the cellular tissue is effected exclusively in the 

 first stage, and, in all probability, is conditioned: 



1 . By the arrangement of the secondary cells in the mother-cells. If 

 they mostly lie in a linear arrangement in the long axis of the stem, an 

 elongated internode originates ; if they lie mostly towards the angles of a 

 tetraedron, an undeveloped internode ; lastly, if they lie chiefly in one 

 plane, this plane may stand at right angles to the axis, and the inter- 

 nodes will be much developed in breadth, or, it may be parallel to the 

 axis, and thus form a stem flattened on two sides. 



2. By the form of the process itself, since this ceases in some situa- 

 tions earlier than in others. 



A. The first distinction to be seized here is that between Monocotyledons 

 and Dicotyledons in general, depending on the division into definite and in- 

 definite, or closed and unlimited, bundles. In the Dicotyledons the process of 

 cell-formation never ceases on the outside of the vascular bundle, whence 

 the individual internodes, so long as they live, continually increase in 

 thickness ; while in the Monocotyledons the process of cell-formation 1. 

 ceases regularly from below upward, in the individual vascular bundles, 

 and thus a thickening of the individual internode by their means becomes 

 impossible ; but the increase of thickness of the axis itself may be 

 attained by the increasing diameter of the successive internodes (as is 

 shown more fully under Z>), and thence, when it rises perpendicularly 

 in a cylindrical form (if it be such as is represented under JB, or under 

 D\ it receives no increase of thickness from that time: or 2. a layer 

 of cells beneath the periphery of the axis retains its capacity of develop- 

 ment, and these continually increase the thickness of the axis by their 

 uninterrupted production of new cells, since in the newly-formed tissue 

 vascular bundles are simultaneously continually developed. This pro- 

 cess occurs, however, only in the Monocotyledons with undeveloped 

 internodes of a branching type, in Draccena, some Palms (Cucifera 

 thebaica\ and Aloinete. This second process of formation bears some 

 resemblance to that of the Dicotyledons, in so far that in both a connected 

 layer of cells remains capable of development around the whole periphery. 

 In both, the newly originating cells assume two forms, one portion join- 

 ing the cellular tissue between the vascular bundles, while another 

 portion belongs to the vascular bundle structure. But the essential dis- 

 tinction remains in this, that this latter portion only increases the 

 existing vascular bundles on the .outside in the Dicotyledons, while in 

 the Monocotyledons, on the contrary, it becomes transformed into new 

 isolated bundles. 



B. If the process of formation progresses regularly from below up- 

 wards, while a definite plane of the basis ceases to produce cells, a cy- 

 lindrical ascending axis is produced. In elongated internodes this is 

 always the case ; therefore every internode may be clearly separated 

 from the axis by two cuts. 



C. If the process of cell-formation ceases somewhat earlier in par- 

 ticular situations in the circumference than in others, the result is 

 the formation of axes with projecting angles ; for instance, three-edged, 



Q 



