504 SECONDARV CHANGES. 



IMamillarieK and Cacte^. and Cobcea scandens, — as certain examples of plants 

 which are constantl}' destitute of annual rings \ 



Whether in tropical trees, as well as in our temperate zones, the annual ring as 

 anatomically distinguished always represents the yea rlj growth, or whether there are trees 

 with half-yearly rings, i. e. such as form two annual rings in the year, corresponding to 

 the two periods of vegetation falling in one annual period, as has been stated to be the 

 case in Adansonia digitata-, appears to me to be a question which ought to be tested, but 

 one that does not belong to the present anatomical exposition. Attention may here, 

 perhaps superfluously, be recalled to the fact that in woods with alternating concentric 

 bands of non-equivalent tissue, as Ficus, Casuarina, &c., markings are present, which 

 resemble annual rings on superficial observation, but from which the true annual rings 

 are distinguished by the characteristic structure of the autumn wood. 



d. Normal differences of successive zones of growth and annual rings. 



Sect. 152. The first innermost annual ring of every wood necessarily shows certain 

 essential differences in its structure from all the later ones. In the stem, instead of 

 the characteristic spring-wood, it has at its internal limit the groups of spiral, annular, 

 and reticulated vessels (or tracheides) corresponding to the primary vascular bundles, 

 but absent from the intermediate bundles ; these usually project more or less into the 

 pith, and, together with the neighbouring tissue-elements, belonging partly to the 

 ligneous ring, partly to the pith, the structural peculiarities of which have been often 

 mentioned in former sections, they have long been distinguished collectively as the 

 medullary sheath or medullary crown, Corona. 



In the root, the internal limit of the first annual ring has a different structure, 

 corresponding to the conditions described on p. 473. The secondary wood here 

 closely surrounds the original axial xylem-plates. It is quite a prevailing rule that 

 these latter remain separated from the secondary vessels and tracheides by at least one 

 layer of connective cells ^. No case is known in which the secondary elements border 

 immediately on the outermost narrow, spiral, and annular vessels of the primary plates. 

 On the other hand, it often occurs that the inner pitted vessels (or tracheides) of the 

 original plates stand in immediate connection with equivalent elements of the 

 secondary wood ; this has been observed in Taraxacum and Ranunculus repens 

 (Fig. 165, p. 356). 



The cells which usually border on the xylem-plates consist of the inner layer of 

 those connective cells which were originally present in this position, while from their 

 outer layer the cambial ring bordering on the phloem-group has been derived 

 (pp. 351, 473). In those cases where the pitted vessels are contiguous, it stOl remains 

 to be investigated whether the latter are derived directly from the above-mentioned 

 connective cells, or from a cambial zone which, in the latter case, must originate in the 

 innermost layer of connective cells. All the tissue-elements of the region in question 

 usually become very thick-walled, and have relatively narrow cavities, especially in 



* Compare Link, Philos. Bot. p. 136. — Meyen, Physiol. I. p. 361.— Treviranus, Physiol. I. 235. 



— Unger, Botan. Zeitg. 1847, p. 267 Schacht, Lehrb. II. p. 62.— Sanio, Botan. Zeitg. 1863, p. 392. 



^ Compare Botan. Zeitg. 1844, p. 367. 

 ^ See Van Tieghem, /. t. 



