INTERNAL ANATOMY OE ORGANS. ROOTS. 533 



parcels divided by secondary medullary rays (fig. 579, B, C), which are 

 repeated in each successive season. The course of the fibro-vascular 

 bundles being slightly sinuous, from their lateral anastomoses, the medul- 

 lary rays have, singly, no great vertical dimensions ; and their transverse 

 diameter varies in different cases. Their cells become lignified in heart- 

 wood. 



Liber. The liber (for an account of the construction of which see ante, 



E. 517) is usually formed in successive thin laminae composed of slender 

 iterally anastomosing bundles of liber-cells; and in some plants these 

 laminae are separated by layers of parenchyma or periderm, so that the 

 liber-structure of old stems may be split into its annual layers. In some 

 stems the liber ceases to grow after the first season. 



The bast of which Russia matting is made consists of the separate 

 liber-layers of the Lime-tree. The " lace " of the Lace-bark tree (Lagetta 

 lintearia) is the liber, and that of other trees of the Order Thymelaceae is 

 used for tying up bundles of cigars &c. 



The Herbaceous Envelope. The cellular or herbaceous envelope (fig. 578, 

 cp) is generally in an active condition of vegetation during the growing- 

 season, since its tissue must increase laterally (tangentially to the stem) 

 to allow of the increasing diameter, while it produces the new suberous 

 structure on the outside. 



Cork-layers. The snberous layer differs much in its condition in dif- 

 ferent trees. The general construction has been alluded to at p. 522. 



The Structure of the Eoot has of late attracted much attention 

 from Nageli, E/einke, Janczewski, Van Tieghem, and others. It 

 is only by studying the mode in which the originally homogeneous 

 cellular mass of the root breaks up into distinct layers and 

 assumes a different form and arrangement of its constituent 

 cells that the structure can rightly be understood. The simplest 

 idea of the root is that of a mere unicellular thread, such as we 

 meet with in the lower Thallogens, and also in the shape of 

 the root-hairs which are produced from the epidermis of more 

 highly organized roots. Physiologically, viewing the root merely 

 as an organ of absorption, this type of root is all important. But 

 in most plants the root is something more than an organ of ab- 

 sorption. It is a laboratory, in which nutritive matters are moved 

 from place to place or are stored up for future use, so that roots 

 of this character have much of the functions of the stem. Ac- 

 cordingly we find, in roots of higher organization, considerable 

 difference of structure, varying also in the different groups of 

 plants. 



The roots of vascular plants may be defined as outgrowths from 

 the interior of the stem provided with a root-cap (pileorliiza\ but 

 as a rule never producing leaves or buds, and growing in length 

 only near the point beneath the cap. 



The general structure of the roots of vascular plants may be 

 defined as consisting of an epidermis, bounding a cortical paren- 



