ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 673 



Formation of Secondary Cortex.* — M. E. Heckel describes the 

 peculiar structure of the wood of a young branch of Sarcocephalus 

 esculentus from Tropical Africa. During the various stages of 

 development of the primitive cortex to the condition of definite 

 secondary cortex, all the initial layers disappear in succession, either 

 by compression or by giving rise, by cell-division and multiplication, 

 to new zones, two of which have a permanent existence. A cortex is 

 thus again formed, constituted definitely of two tissues of secondary 

 or even tertiary formation, entirely destitute of liber, the place of 

 which is taken, from a physiological point of view, by certain elements 

 which have become sclerotized. The author believes this structure to 

 be not uncommon in tropical woods. 



Pericycle of the Root, Stem, and Leaves.f — According to M. L. 

 Morot, there exists in all flowering plants, outside the central cylinder 

 of the root, between the endoderm and the outermost part of the 

 fibrovascular bundles, a layer of tissue, of the same origin as the pith 

 and the medullary rays — the pericycle. It is the most important 

 part of the internal conducting system of the root, from the secondary 

 and tertiary formations to which it gives birth, and from the frequent 

 absence of the pith in consequence of the fusion of the primary 

 bundles, and also of the medullary rays. Although most often re- 

 duced to a single layer of cells, it sometimes constitutes a layer of 

 considerable thickness. It is usually homogeneous, but sometimes 

 contains secreting canals. It is from the pericycle that the secondary 

 roots always proceed ; and it may, by the repeated divisions of its 

 cells, produce cork, secondary parenchyma, and secondary or tertiary 

 fibrovascular bundles. 



The presence of the pericycle is nearly as invariable in the stem, 

 where it may also persist in the absence of the pith and medullary 

 rays. The only instances of its being entirely wanting are in certain 

 aquatic i)lants of degraded structure. Occasionally it forms a separate 

 envelope round each bundle ; in the vast majority of cases it consti- 

 tutes a continuous sheath round the whole of the central cylinder. 

 It usually consists of a number of layers of cells, but is sometimes 

 reduced to only one. Its structure is more complex than in the root. 

 Sometimes it remains entirely parenchymatous ; but it is most often 

 partially sclerotized, and then contributes largely to the constitution 

 of the storeome of the stem. In addition it may inclose laticiferous 

 vessels, resinifertjus cells, and secreting canals. Like that of the 

 root, the pericycle of the stem may generate new tissues. From it 

 proceed underground or aerial lateral roots; or it may develope 

 intercalary vascular bundles between the primary ones, cork, secondary 

 parenchyma, and centrifugal layers of meristem. 



The pericycle occurs equally in the leaves, where it is inclosed, 

 like the endoderm, between the bundles, rarely forming a complete 

 ring round them. Its composition varies greatly as in the stem ; it 



* Bull. Hoc. Bot. Franco, xxxii. (1884) |.p. 0.5-9 (1 pi.), 

 t Ann. Sci. Nut.— i'.ot., xx. (188.0) pp. 'Zll-WJ ((J jils.). 



