1872.] 



Plants of the Coal-measures, 



201 



that cylinder is an additional element which has no counterpart amongst 

 the living forms of this group. 



Though the central compound cellulo-vascular axis continued to in- 

 crease in size with the general growth of the plant, it was always small in 

 proportion to the size of the stem. The chief enlargement of the latter 

 was due to the growth of the bark, which exhibited three very distinct 

 layers, — an inner one of cells with square ends, and slightly elongated verti- 

 cally and arranged in irregular vertical rows, an intermediate one of 

 prosenchyma, and an outer one of parenchyma. These conditions became 

 yet further modified in old stems. The exogenous ligneous zone became 

 very thick in proportion to the medullary vascular cylinder, and the dif- 

 ferences between the layers of the bark became yet more distinct. These 

 differences became the most marked in the prosenchymatous layer ; at its 

 inner surface the cells are prosenchymatous, but towards its exterior they 

 become yet more elongated vertically, their ends being almost square, 

 whilst numbers of them of exactly equal length are arranged in lines 

 radiating from within outwards. These oblong cells often pass into a yet 

 more elongated series with somewhat thickened walls, which become 

 almost vascular, constituting a series of bast-fibres. In the transverse 

 sections these prosenchymatous cells are always arranged, like the vessels 

 of the ligneous zone, in radiating lines. Yet more external is the sub- 

 epidermal parenchyma passing into leaves composed of the same kind 

 of tissue. The petioles of the leaves have been long, if not permanently, 

 retained in connexion with the stem, a character of Corda's genus Loma- 

 tophloios. 



Where young twigs branch, the vascular medullary cylinder divides 

 longitudinally into two parts ; the transverse section of this cylinder now 

 resembles two horse -shoes pointing in opposite directions. The break in 

 the continuity of each half of the cylinder occasioned by the division is never 

 closed by new vessels belonging to the cylinder ; but when the stem de- 

 velops exogenously, the cambium-layer, from which the new growths 

 originated, has endeavoured to surround these openings in the cylinder, 

 and, by closing them, once more to separate the medullary from the cor- 

 tical tissues. Some beautiful specimens have been obtained, which exhi- 

 bit these new exogenous layers in process of formation. The vessels or 

 the young layers are not half developed. At first they meander vertically- 

 through masses of delicate cellular tissue ; but they soon arrange them- 

 selves in regular radiating vessels and cells, becoming mere outward pro- 

 longations of the woody wedges and medullary rays of the older part of the 

 stem. At this stage of their growth, the walls of the vessels are deeply 

 indented by the contiguous cells, as if the plastic tissues of the former had 

 been moulded upon the latter structures. As the new vessels enlarge, the 

 superfluous intervening cells disappear, until each medullary ray finally 

 consists of a single vertical pile of from one to a small number of cells, 

 arranged as in many Coniferoe. The exceptional cases are those where 



