18G Prof. Henfrey on the Development of Roots, 



ragged appearance, which probably gave rise to the idea of a 

 spongy structure at the end of the rootlets. 



" In some roots the epidermis produces no fibrils, but remains 

 smooth. This is especially the case in the delicate filamentous 

 roots, annually thrown off, of many Monocotyledons, as of the 

 onion, hyacinth, crocus, &c. In these roots the epidermal cells 

 retain their general delicate character throughout their existence; 

 and probably the roots of this character absorb by their surface 

 throughout their whole length ; while in woody roots the ab- 

 sorbent action is confined to the rootlets — to the regions near 

 the growing points, — where the epidermis is still delicate and 

 covered with its hair-like fibrils. 



" In woody roots, as the whole organ increases in size and the 

 internal part becomes lignified, the cortical region changes its 

 character. The epidermis dries up, and its place is taken by a 

 corky structure, formed of two or three layers of the cells pre- 

 viously subjacent to the epidermis. When this change has taken 

 place, the direct absorbent power may be regarded as lost. 

 Simultaneously with this change, the inner cortical parenchyma 

 often increases considerably in quantity, and this is particularly 

 the case in fleshy roots, where this region subsequently becomes 

 the reservoir of accumulated nourishment. 



" The centre of a very young root is occupied by a cord of 

 cellular tissue of different form from the cortical parenchyma, 

 consisting of elongated cells — the cambium of the future wood, 

 which merges, near the growing point, in the focus of cell- 

 development lying just behind the apex of the rootlet, where 

 the nascent cells are all alike. The central cord very soon dis- 

 plays traces of the structures called ducts, and the cells assume 

 the form, and more or less the substance, of the wood-cells of 

 the stem. Some important differences exist as to the arrange- 

 ment of their constituents in different classes of plants. In 

 Dicotyledons (such plants as turnips, beans, pease, our native 

 timber-trees, &c), the structure of the central or woody part of 

 the root differs from that of the stem chiefly in the absence of a 

 central pith, together with the circumstance that the so-called 

 vascular structure consists of short-jointed ducts, without the 

 more flexible spiral vessels. 



"In ordinary Dicotyledonous roots, when no tuberous deve- 

 lopment occurs, the central woody structure soon acquires its 

 distinctive character. The wood of the stem consists originally of 

 a number of perpendicularly arranged cords, standing in a circle 

 around the pith, a certain number of which pass out into each 

 leaf to form the skeleton of those organs. The lower portions, 

 inside the stem, extend down for a variable distance in different 

 plants. Those of the lower joints of the stem run down into the 



