STRUCTURE] ANOMALOUS STKUCTURES. 235 



nodes occurs as an exceptional case in those Palm-stems 

 which are swollen in the middle. Those who would study 

 this form of stem in Monocotyledons and have not Palms 

 at hand, may take Allium strictum and senescens, &c., 

 which are in reality, Palm-stems in miniature. 



IV. Other varieties of dicotyledonous structure result from 

 the excessive enlargement (hypertrophy) of the pith, or bark, 

 or both, as in Spurges, (Euphorbise,) Indian Figs (Cactacese), 

 many tubers, as the Potato, and particularly in Cycads, 

 whose stem has only a superficial resemblance to that of 

 Palms, and is much more nearly allied to that of Fern-stems, 

 differing, however, essentially in its unlimited woody bundles, 

 and approaching far more to Indian Figs. 



V. Finally, the nature of the cells which compose the 

 woody bundles, either originally or eventually, differs much 

 more than has been hitherto believed. The light wood of 

 Avicennias consists almost wholly of porous tissue. The 

 light and soft wood of Bombax pentandrum, of parenchym, 

 spiral, annular, and reticulated vessels ; but rarely of pros- 

 enchym, in the outside of its annual rings. The wood of 

 the Melocacti, Mammillarise and Echinocacti consists entirely 

 of short, broad, thin-walled cells, terminating above and 

 below in an obtuse cone, and having very thick rings or 

 spires attached by their narrow edge, like those which 

 Meyen has represented in his Phytotomie, from Opuntia 

 cylindrica, where they occur, as in most of the Opuntia^ 

 though in less abundance, at the contractions of the joints. 

 It is well known that in Conifers and Cycads the cells which 

 form the wood are all alike, and do not consist as in many 

 other kinds of wood, of prosenchym, and vessels. In many 

 plants the first formed spiral vessels of the medullary sheath 

 become changed by their great extension in length, into 

 annular vessels, in which form they are left ; in other plants 

 the spiral vessels do not show this tendency, notwithstanding 

 the great extension they undergo; in such cases they are 

 frequently lengthened to such a degree as to resemble a mere 

 thread lying in an intercellular passage, and they are 

 frequently entirely re-absorbed. This may be beautifully 

 observed in Opuntia monacantha, cylindrica, Mammillaria 



