ZOOLOGY AND BOTANY, MICROSCOPY, ETC. Ill 



Exogens are of endogenous origin, from the deeper tissues of the stem ; the 

 CrucifersB furnish an exception, the lateral roots being exogenous in that 

 order. It is most frequently the case in Dicotyledons that the lateral roots 

 are formed at the expense of a layer of cells at the periphery of the central 

 cylinder of the stem itself formed from the pericycle. The roots are 

 sometimes formed at the portion of the pericycle which faces the vascular 

 bundles, sometimes at the side of the bundles, while sometimes they spring 

 from the intervals between the bundles. When the pericycle is simple, it 

 first divides into two layers by tangential walls; the lower layer gives 

 birth to the central cylinder of the root ; the superficial layer again divides 

 into two, the inner layer producing the cortex, the outer the root-cap 

 and the piliferous layer. When the pericycle is composed of several 

 layers, the cells of the internal layer form the central cylinder of the root, 

 while the cortex, the piliferous layer, and the root-cap spring from the 

 outer layer. 



The endoderm or the last layers of the cortex in some cases take their 

 share in the development of the root. It gives place to a tissue (the calotte), 

 composed of one or more layers which clothe the root-cap. Sometimes 

 the cells of the endoderm, full of protoplasm, divide in the radial direction, 

 and produce a layer of cells extending to the surface of the root-cap ; or 

 the endoderm first developes round the young root a layer which sub- 

 sequently divides into two by tangential walls ; or several inner layers of 

 cortex are associated in producing several layers of " calotte." In some 

 plants the cortex of the root is altogether inactive. 



In plants belonging to the Leguminos® the pericycle of the stem gives 

 birth only to the central cylinder of the root, while the other parts are 

 derived from the last layers of the cortex of the root. In some cases 

 again ( Yinca major, Viola palustris and odorata) the roots are not formed 

 at the expense of the pericycle, but from a meristem situated within the 

 liber of the bundles of the stem, in other words, from the intrafascicular 

 cambium. In Asperula odorata the central cylinder of the roots proceeds 

 from a generating layer beneath the liber ; the other parts are produced 

 from the pericycle. In the CrucifersB the lateral roots are exogenous ; 

 their central cylinder is the result of divisions in the second layer of the 

 cortex of the stem ; their cortex results from segmentations in the outermost 

 layer of this same cortical tissue ; the piliferous layer and root-cap have 

 a common origin, viz. the epidermis of the stem. 



Aerial Roots of Soimeratia.* — Herr K. Goebel has examined the roots 

 of this tree, growing in tropical swamps, and has determined that they 

 are not pathological structures, as previously supposed, but normal roots 

 which emerge from the water or mud owing to their negative geotropism. 

 He regards their function as being connected with respiration, to bring 

 the roots in direct contact with the atmosphere in consequence of the 

 small amount of oxygen contained in the mud. 



Structure and Function of the Subterranean Parts of Lathraea 

 squamaria.f — Mr. G. Massee describes in detail the structure of the scale- 

 leaves and " haustoria " of this plant, which he regards as a saprophyte 

 rather than a true parasite. The haustoria or discs, by means of which 

 nutriment is obtained from the host, are about a line in length, are best 

 developed on the primary rhizome and its branches, and may be terminal or 

 interstitial ; sometimes they are so numerous as to give a moniliform 

 appearance to the rootlets; but are sometimes almost altogether absent 



* Ber. Deutsch. Bot. Gesell., iv. (1886) pp. 249-55. 

 t Journ. of Bot., xxiv. (1886) pp. 257-63 (1 pi.). 



