February 1, 1894.] 



KNOWLEDGE 



45 



intervals, and forming a kind of internal framework. By 

 scraping awa}' the softer substance, a beautiful hollow 

 cylinder of network, the fibro-vascular system of the fern, 

 is exposed. This system, present in all the higher plants, 

 assumes characteristic forms in the leading divisions of 

 the vegetable kingdom. In the stem of a palm or other 

 monocotyledon the fibres are irregularly scattered through 

 the cellular substance ; the softer part being in the centre, 

 this arrangement is termed endogenous. The stems of 

 dicotyledons and conifers, on the other hand, are exogenous 

 and show a central pith surrounded by rings of wood and 

 layers of bark. The young exogen has a ring of five 

 bundles or so, reminding us of the adult condition of the 

 fern stem, but every succeeding year a fresh ring of 

 secondary bundles is formed towards the outside ; the 

 primary bundles at the same time gi'eatly increase in 

 diameter, so that very soon the vascular system in the 

 exogen assumes much greater importance than it has Ln 

 the fern, and ultimately constitutes the great bulk of the 

 stem. Cellular ground tissue occupies all the space en- 

 closed by the epidermis which is not taken up by the 

 vascular framework. The ground tissue includes the pith, 

 most of the bark, and the medullary rays. The last 

 mentioned are vertical plates of tissue which pass from the 

 pith to the bark, separating the bundles, and penetrating 

 the meshes of the vascular system ; to them is due the 

 appearance known as the silver grain of wood, easily 

 recognized even in fossil specimens. The primary vascular 

 bundles formed during the first season are the only ones 

 ever developed in ferns. Dicotyledons and Gymnosperms 

 are the onlj' living plants in which secondary thickening 

 occurs. By this is meant the increase of thickness that 

 occurs after the part has ceased to grow in length. A 

 certam length of stem is produced during a season of 

 growth. In ferns and palms this portion never becomes 

 any thicker ; the trunk of an exogen like the oak, on the 

 other hand, increases in girth as long as it lives. With 

 the single exception of Isoetes, in no existing fern, club- 

 moss, horsetail, or other Cryptogam does secondary 

 thickening take place. The extinct Lepidodendron and 

 Calamite differed in this respect very widely from their 

 modern representatives. Williamson has shown that they 

 possessed the power of forming secondary wood by 

 exogenous growth. The loss of this power may perhaps 

 account for the diminutive proportions of existing Lycopods 

 and Equisetacciie, to which the arborescent Lepidodendron 

 and Calamite of palfeozoic times were closely allied. 



The separation of the secondary wood of exogens into 

 rings is due to a difference between that formed in autumn 

 and the portion added in spring ; the autumnal wood is 

 dense, the vernal is of looser texture and consists of larger 

 and more delicate cells. This is, however, only the case 

 in temperate climates where the seasons are marked. In 

 tropical woody plants annual rings are not, as a rule, 

 distinguishable. As indicating the former extension over 

 the globe of a more uniform and tropical climate, it is 

 interesting to note the entire absence of yearly rings in the 

 fossil woods of the palieozoic strata. 



The stem of Lepidodendron, though agreeing with that 

 of an ordinary exogenous tree as regards the formation of 

 secondary wood, was in other respects very dissimilar. 

 In succulent and herbaceous plants the ground tissue 

 preponderates over the vascular system, and Lepidoden- 

 dron partook somewhat of the herbaceous character. 

 There was a central axis or cylinder of small diameter 

 relatively to the stem, consisting of a ring of wood which 

 varied in thickness with age. Inside, the woody ring 

 enclosed an enormous pith ; outside, it was surrounded 

 by a bark or cortex of great thickness. The bark of most 



trees is thin compared to the woody part, but the rind of 

 Lepidodendron occupied in many cases quite four-fifths of 

 the diameter of the stem. It consisted of three layers ; 

 the innermost had a delicate spongy character, and is 

 rarely preserved. Of this tendency of the inner cortex to 

 degenerate we have an extreme example in the modern 

 Selaginella, in which the central axis is separated from the 

 cortex by a vacant space traversed here and there by 

 delicate horizontal cells or trabeculfe. In various members 

 of the Lycopod family. Prof. Bower has recently found this 

 inner layer of the rind to exhibit transition stages inter- 

 mediate between the conditions in Lepidodendron and 

 Selaginella. To compensate for the weakness of the 

 vascular system, the outer cortex in many extinct 

 Cryptogams was strengthened by strands of schlerenchyma, 

 consisting of thick-walled cells forming the dictyoxylon- 

 rind of Brongnart. A fossil stem in the writer's 

 possession shows, in addition to the woody cylinder, 

 scattered vascular bundles running up through the thick 

 pith. Under the name Heterangium, Williamson has 

 described a fossil plant with a similar arrangement. 

 This condition, which combines the characters of endogen 

 and exogen, is very exceptional among existing plants ; 

 species of Aralia and Phytolacca are among the few 

 examples known. Williamson is also of opinion that in 

 some lepidodeudroid plants secondary thickening occurred 

 within the pith, a peculiarity unknown in living plants. 



A longitudinal section of a stem under the microscope 

 in all cases shows the woody bundles to be made up of 

 elongated tubes and cells, the walls of which exhibit dots, 

 rings, spirals, and various other marks. In ferns and 

 Lycopods the cell-walls are crossed by horizontal bars 

 giving the ladder-like or scalariform character. Wood 

 vessels are formed by the fusion of vertical rows of cells. 

 Some of the larger of these, from the resemblance which 

 their ring-like marking gives them to the windpipe of an 

 animal, are named tracheae. Strings of cells in which the 

 partitions still remain also occur ; such cells are the 

 tracheides, so-called because of their likeness to the true 

 vessels or tracheie. Vessels proper are most abundant in 

 the primary bundles forming the oldest wood next the 

 pith ; the bulk of the secondary wood, in conifers almost 

 the whole of it, consists of tracheides. These are elon- 

 gated tubular or prismatic cells, with oblique or tapering 

 ends which dovetail with those of the cells above and 

 below, and are distinguished as pitted, scalariform or 

 spiral, according to their markings. Tracheides are not 

 uncommonly six-sided, giving to the wood a columnar 

 structure resembling on a small scale the basaltic pillars 

 of Fingal's Cave or the Giant's Causeway. The arrange- 

 ment of the tracheides often reminds one of a collection of 

 organ pipes. The wood of the common arbor vit:e has an 

 exceedingly beautiful structure ; the tubular tracheides 

 resemble so many glass flutes with the row of pits corre- 

 sponding to the stops of the instrument. Crossing the 

 transparent flutes, as if to bind them together, at intervals 

 are seen the rows of brick-like cells so characteristic of 

 medullary rays. Tracheides and medullary rays cross 

 each other very much as warp and woof do in cloth, to 

 which the texture of wood has some analogy. 



Coniferous wood is easily recognized ; its constituent 

 tracheides exhibit the characteristic markings known as 

 bordered pits. Each pit presents the appearance of a 

 small circle surrounded by a larger one ; the shape and 

 arrangement of the pits vary, however, in different 

 groups of ConifersB. Giippert recognized four, but Krauss 

 distinguishes six distinct types of fossil wood. Taxites, 

 represented by the modern yew, has only been obtained 

 from Tertiary rocks ; the cypress type is found in the 



