186 Mr. A. Henfrey 07i some Points in the Structure 



There is another point in the structure of Monocotyledons 

 which does not appear to me to be clearly understood. In 

 the cortical region are often found fibrous bundles, passing into 

 the leaves above, and which, in the cases I have examined, pos- 

 sess no vessels, and consequently have the character of liber 

 bundles. In Sparganium ramosum they are very numerous and 

 of considerable size; they are also found in the Grasses, and 

 apparently in all such stems as have the internodial portions 

 developed. I cannot find them in bulbs, or in such rhizomes as 

 the common Iris; but in the flowering-stem of many such 

 plants, ex. gr. the Tulips, Crown Imperial, Tiger Lily, &c., the 

 parenchyma gradually changes its character towards the peri- 

 phery, and just within the epidermal layers it consists of long 

 fibrous cells exactly resembling liber cells. This ring of woody 

 cells is very much developed in Tamus communis and Smilax 

 aspera (and I believe in most plants belonging to the same orders), 

 and has been taken for a layer of wood analogous to that of Di- 

 cotyledons ; but this is certainly a misconception, as it is di- 

 stinctly defined at its outer border, and does not present the 

 slightest trace of a developing or cambium layer. Internally it 

 passes insensibly into the central parenchyma of the stem, in 

 which lie the true woody bundles having vessels as usual. If 

 this has an analogue at all in Dicotyledons, it is the liber ; and 

 there are many reasons in favour of such an hypothesis, espe- 

 cially if we consider it as identical with the fibres of the cortical 

 layer of such stems as Sparganium. 



In conclusion, advertmg to the discussions as to the mode of 

 increase of the stem of Monocotyledons, it seems to me that 

 many writers do not sufficiently consider the peculiar characters 

 of this organ. This question must not at all be mixed up with 

 that of the formation of new layers of wood in Dicotyledons, 

 since in all the usual forms (making exception of such as 

 Draccma, Cordyline, &c.) nothing analogous to the second year's 

 layer of wood is ever produced. In bulbs the buds are thrown 

 off; in branching rhizomes they become independent physiolo- 

 gically, and the old stem never increases; while in the Palms, 

 the bud, though continually developing, merely applies a new 

 layer spirally upon the pre-existing portions, so that the new 

 growth might be roughly exemplified by a series of hollow cones 

 of equal size placed one upon another. Here all growth is ana- 

 logous to the upward growth of the terminal bud of a dicotyledon ; 

 there is no cambium layer, and no peripherical increase*. The 



* For every year's development of the terminal bud of a dicotyledon we j 



find a new layer deposited all over the old wood by the cambium layer, form- j 



ing a new annual ring ; in Palms we have merely the terminal shoot with- '•. 



out any analogue of the annual ring. { 



