406 Dr. G. Ogilvie on the Forms and Structure of Fern-sterns. 



by the interlacement of the longitudinal fibro-vascular bundles. 

 Though the formation of wood in Dicotyledonous plants is 

 spoken of as being on the exterior, and has hence given rise to 

 the stem being termed Exogenous, still it is to be remembered 

 that at the growing points the fasciculi do in fact pass from the 

 petioles of the leaves into the interior of the stem, and only gain 

 that position which characterizes the rest of their course by 

 curving outwards again to apply themselves to the exterior of 

 those of older formation. 



In the Monocotyledonous stem, again, the fibro-vascular 

 fasciculi, though they tend at last in the same way to place 

 themselves on the outside of those of older development, lose 

 themselves here, by the dispersion and occasional anastomosis 

 of their elements, in what has been termed the fibrous layer 

 of the stem, and do not descend any way on the exterior, 

 or form any new continuous envelope like an annual layer of 

 Exogenous wood. Hence, should we wish to represent the in- 

 crements of a Monocotyledonous stem by a series of superposed 

 truncated cones, we must make them, not, as in Dicotyledons, 

 of continually increasing dimensions, so that the outer shall 

 completely envelope the inner, but we must make them all of 

 the same size, so that the series as a whole may have, not a 

 conical but a cylindrical outline, the outer cones being borne up 

 by those within and below them, whose bases they can no longer 

 cover. They are also more truncated or more open at the top, 

 from the larger size of the cellular core of the terminal bud. 

 Such a stem, on horizontal section, is described as presenting 

 three regions: — 1st. A central, which in some respects corre- 

 sponds to the pith of a Dicotyledon, but which contains im- 

 bedded in the cellular tissue the ends of numerous fibro-vascular 

 bundles, divided in their descending but at the same time out- 

 ward-bound course through the interior of the stem. 2ndly. A 

 cortical zone, also of cellular tissue, differing from the bark of a 

 Dicotyledon in not being generally separable from the stem, and 

 in rarely containing any fibrous tissue. 3rdly. Intermediate 

 between the central and cortical regions, a ring of densely-matted 

 fibrous tissue, formed by the anastomosis of the lower and outer 

 ends of the fibro-vascular bundles. 



In the stems of Ferns, the fibro-vascular element is but very 

 sparingly developed in proportion to the cellular, and the dis- 

 position of the fasciculi assumes in consequence a different 

 appearance from that in either of the higher groups ; the vas- 

 cular bundles are well developed in the petioles of the fronds, 

 but they enter no further into the structure of the stem than 

 simlpy to effect a union with those derived from the fronds of 

 former years. They have no downward course in the stem ; the 



