THE STEUCTUEE OE THE EOOT 13 



-'*'■■, 



fibres. On account of their siniflarity -with the Vessels, how- 

 ever, which are sometimes called traehese, they have been 

 termed tracheids, and they are further characterised by a 

 curious form of pores termed bordered pits. The tracheids 

 perform the same, function as the vessels of other plaDrts. 

 They conduct the nutritive salts dissolved in water from the 

 soil first into the leaves, where, under the influence of the 

 light and together with the carbonic acid of the air, they are 

 transformed into organic food matter. The other portion 

 of the fibro-vascular bundle lies towards the outside, and 

 consists of flexible, thin-walled sieve-tubes and cambiform 

 cells rich in protoplasm. This soft portion of the bundle, 

 which is concerned in the conduction of the organic nitro- 

 genous food substance, and is termed the soft bast, is generally 

 protected on the outside by strands of thick-walled hard bast 

 cells (b). 



These bundles, whether they consist of real vessels or 

 tracheids, conduct along their woody portion the raw materials 

 which the roots have absorbed, and carry them into the finest 

 ramifications of the stem. Through their soft bast tubes, on 

 the other hand, they conduct the elaborated substances which 

 are necessary for the increase in root and shoot from the 

 leaves, where they are formed, to the root-tips. As these con- 

 ducting cells are very delicate, and might easily be compressed 

 and damaged by any external pressure, we find them generally 

 protected on the outside by a band of hard bast cells.^ 



Let us now examine also in cross-section the structure of 

 the root which we have studied from a longitudinal section. 

 We choose as an example the root of . a monocotyledonous 

 plant, namely, that of the Shallot (Allium ascalonicum) (Fig. 3), 

 and a root of a dicotyledonous plant, that of the Auricula 

 (Frimwla Auricula) (Pig. 4). In both figures the soft cortical 

 tissues, which are of secondary importance in the present con- 

 sideration, as well as the epidermis and root-hairs, have been 

 left out, and are only indicated by a few layers of cells at the 



' These protective strands of hard bast have been considered as part of the 

 vascular bundle, and the bard and soft bast together have received the name 

 of phloem, while the wood vessels together with the wood fibres have been 

 called the a^7em. BothTpaTta'togetheT ioiva the Jibro-vasoidar bundle. 



