INTERNAL STRUCTURE OF STEMS. 



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stem the cords are scattered about, apparently without any order, as may 

 be seen in a stem of Asparagus. 



The central cylinder of the stem of the former commences with the 

 pericycle immediately under the endoderm of the cortex. The cords 

 abut against it. Each cord consists of phloem, cambium, and xylem. 

 The first is composed of "sieve-tubes" and "companion-cells." The 

 cambium is the permanently active layer, and then follows the wood, 

 consisting of wood-fibres and vessels ; within is the soft pith or medulla, 

 this and the pericycle being connected by medullary rays. In a mono- 

 cotyledon, as the cords are isolated, the cambium in each becomes useless, 

 as it cannot form a united cylinder of wood, so that the stem, as a rule, 

 cannot increase in diameter after it has attained its full size. 



The pericycle, however, often undertakes some active work in com- 

 pensation for the loss of the cambium, as in those liliaceous trees like 

 Draccena, which branch. In these the stem can increase in size. 



The pericycle often plays an important part in herbaceous as well as 

 woody stems. Thus, in all which are characterised by having a fibrous bark, 

 as Hemp, Flax, Vine, Clematis, Honeysuckle, &c, it is this layer which 

 makes fresh quantities of fibre every year in such as are perennials. The 

 result is that the whole of the cortex outside of the pericycle is sometimes 

 thrown off, and this originally deep-seated layer supplies the dead fibres 

 which appear as a ragged coat on the surface of older shoots and stems. 



In many annual flowering stems of monocotyledons, the pericycle 

 gives great rigidity ; for all the cords seen in a cross section are included 

 within the pericycle ; just as in a dicotyledonous stem, though situated 

 irregularly in the monocotyledon. The pericycle may then form a dense 

 sclerenchymatous sheath, giving great stiffness to such flower-stalks as of 

 the Lily of the Valley, Ixias, &c. 



Another use of the pericycle which is generally present in roots, 

 though often in abeyance in stems, is to produce the secondary and other 

 rootlets and root-fibres. These always arise from the pericycle, making 

 their way through the overlying cortex by dissolving the tissue, and in 

 fact living upon it, until they make their exit at the surface. This absorb- 

 ing process is done by a " pocket " over the top which secretes a ferment 

 capable of dissolving and digesting the mother-tissues in front of it. 



In Tree-ferns a somewhat different arrangement and structure of the 

 cords exist, in that, while being for the main part of their length 

 separate, yet they join at intervals, so that if the whole could be isolated 

 the cords would form a sort of network. 



Very anomalous structures are seen in woody climbers, as the Lianes 

 of tropical forests. These arise in consequence of the various strains 

 to which their enormously long stems are inevitably subjected. Hence 

 they must be flexible, elastic, and tough, acting as they do precisely like 

 powerful cables. 



These qualities are secured in various ways, as by a great increase in 

 the size of the medullary rays and by their being almost like cork, in 

 Bignonias. In others longitudinal ribs are produced, and the whole stem 

 becomes twisted so that it closely resembles a cable of many strands. 

 Such occur in the members of the order Malpighiacece. 



In fact, the anomalies of woody climbers are innumerable. 



