188 Prof. Henfrey on the Development of Roots. 



the pith of the stem, although it bears some resemblance to it. 

 There is a distinct boundary of wood where the root joins the 

 stem. This is probably of importance as regards the 'keeping' 

 qualities of the roots. In the carrot there is a similar develop- 

 ment in the woody region, but not so marked ; while an equal, 

 if not greater, production of parenchyma takes place on the outer 

 side of the cambium, forming a thick fleshy rind. A thickened 

 rind of this kind is found in most of the fleshy fibrous roots of 

 perennial herbaceous Dicotyledons, such as groundsel, prim- 

 rose, &c. 



" In the beet, the structure both of the stem and root is un- 

 like that of ordinary Dicotyledons ; and the changes produced by 

 cultivation cannot be discussed here. 



"The roots of Monocotyledonous plants, such as those of 

 grasses, onions, ordinary bulbous plants, &c, are temporary 

 structures, thrown off year after year, or dying with the stem in 

 annuals. Their woody structure differs very much from that of 

 the roots of Dicotyledons, so that they are easily known by ob- 

 serving a cross section ; but the cortical region and the growing 

 extremities differ little in the roots of the two classes. The 

 principal characteristic of the roots of the Monocotyledons lies 

 in their woody central cord exhibiting no trace of distinct bun- 

 dles separated by medullary rays, but consisting of a central 

 column of wood, with its 'ducts' or vascular structures lying 

 on the outside, at the region where the wood adjoins the cortical 

 parenchyma. A kind of cambium exists here also, although no 

 annual rings are ever formed, since it is at this outer surface 

 of the woody region that the root-buds originate. 



" The structure of the ordinary roots of herbaceous Mono- 

 cotyledonous plants may be well examined in the onion. If we 

 place an onion bulb over water in a long glass, like a hyacinth- 

 glass, it soon sends out a number of slender blunt-ended roots, 

 of white colour, the tips only having a yellowish tinge. By 

 placing longitudinal sections of one of them under the micro- 

 scope, we can trace the mode of development of their roots. 

 The extreme point is clothed by irregularly formed cells, loosely 

 coherent, and evidently being partly thrown off by expansion of 

 the structure beneath ; these cells pass laterally into a stratum 

 of elongated cells, which clothe the whole external surface of the 

 rootlet. In the interior of the conical end of the root we find a 

 mass of nascent cells, with their walls scarcely distinguishable, 

 in a state of rapid multiplication by division : this is the chief 

 focus of development of the root. Continuing the examination 

 upward to the older part of the root, the rudimentary cells are 

 soon found arranged in rows parallel to the direction of the 

 root : at first they are very short, then squarish in the side view ; 



