588 SECONDARY CHANGES. 



In the young branch of Cocculus laurifolius the narrow pith is surrounded by a normal 

 ring of wood and bast, as in Menisperraum (p. 456), with broad medullary rays, and rela- 

 tively narrow bundles. Each portion of bast of the latter is limited from the outer cortex 

 by a many-layered half-ring of hard sclerotic fibres. The outer cortex is chiefly paren- 

 chymatous, about 7-9 layers of cells thick, including the epidermis, and without special 

 peculiarities. The normal growth in thickness of the first zone of wood and bast con- 

 tinues, as far as is known, for 1-2 years. Then the activity of the cambium ceases, A radial 

 extension now begins in the 2-3 inner layers of parenchyma of the outer cortex, followed 

 by tangential divisions, the latter proceeding especially, though not exclusively, in a cen- 

 tripetal direction. The thickness otthe parenchymatous cortex thus increases to i8-jo 

 layers of cells. Of these the 10-11 outer ones remain unaltered : 3-4 layers next within 

 these become hard stony elements, forming a continuous ring. Inwards from this ring 

 the tangential divisions continue, especially in centrifugal progression. Some of the 

 inner layer of cells thus formed pass over into the condition of permanent very thick- 

 walled parenchymatous cells ; they form a continuous zone surrounding the exterior of 

 the primary strands of bast-fibres ; into this the primary medullary rays, which resemble 

 it in structure, are directly continued. Outside this layer there remains a ring of 

 meristem at first only 1-2 layers of cells thick, in which the second thickening ring is now 

 differentiated. The structure of this is in all respects very similar to that of the first ; 

 but its bast-bundles, which are almost semicircular in transverse section, have no fibrous 

 sheath. After the cambiogenetic increase of the second ring has continued for a time, 

 the activity of its cambium ceases; between the outer limit of its bast and the stony ring 

 appears a new thickening ring of the same sort as the second, and so successively 

 onwards. 



The figure 233 gives a diagrammatic view of the real appearance of the transverse 

 section of the stem described, and of its congeners, though the representation is not exact 

 since it is taken from another plant. 



In the structure of the stem of the Avicennias (A. officinalis, nitida, tomentosa) the 

 soft bast and cambium were not clear in the dry material at my command, I therefore 

 limit myself to short notes. The first normal ring of thickening surrounds a wide pith: its 

 irregularly undulating outer margin is marked off by a ring of bast fibres, which shows 

 but few interruptions, from the usually delicate parenchymatous outer cortex, which con- 

 tains scattered, short, and elongated sclerenchymatous elements. The wood shows a 

 normal Dicotyledonous structure, and is arranged in very regular radial rows. The 

 usually pluriseriate medullary rays consist of rather thick-walled cells, and are continuous 

 with this structure at many places as far as the ring of bast sclerenchyma. Thus as 

 thick-walled bands they divide up the bast zone, which consists for the most part of very 

 delicate elements with isolated fibres scattered here and there, into well-marked sections 

 of unequal size. The first ring hardly attains a thickness of ^^m. Then follows a second 

 ring of thickening, which abuts directly within on the above-mentioned fibrous zone, and is 

 limited externally by a zone of sclerenchyma on an average 1-2 layers thick the elements 

 of which resemble those of the fibrous zone in transverse section, but are shorter than 

 they. To this zone a third, and then numerous successive thickening rings are added, 

 always in the same manner. The later ones also remain narrow ; in a branch s"™ thick 

 for instance they are not on the average thicker than 0-5™™. 



The structure of all successive rings is just like that of the first : also the interruption 

 of the soft bast by thick-walled radial bands always takes place, and in such a way that it 

 is effected not only by medullary rays, but here and there by strands of wood, with few 

 vessels, which extend to the outer ring of sclerenchyma. The soft bast is often broken 

 up into single strands which resemble the strands of sieve-tubes of Strychnos. At the 

 time when the youngest ring appears the growth in thickness of the next inner one has 

 in each case already ceased. As regards the origin of the rings it was only possible to 

 make out that a zone of meristem appears outside the sclerenchymatous limit of the last 

 mature ring, in the deUcate parenchyma of the outer cortex. When this zone is still 



