ON THE STRUCTURE OF THE ROOT. 73 



part of the root where the root-hairs are given off. We thus see 

 that in the region of the root-hairs water can easily pass through 

 the endodermis from the outside, but that in the higher regions of 

 the root the complete cuticularisation of the endodermal cells 

 prevents the passage of water inwards or outwards, so that the 

 water taken into the central cylinder cannot escape. 



Inside the endodermis we see that the cells of the central 

 cylinder are grouped into masses or strands, and we shall find that 

 each of these groups of cells has a certain function to perform. 

 (A group of cells which performs any special function is called a 

 tissue.) First, we shall notice six distinct bundles of cells, which 

 possess very thick walls, and can be recognised quite easily, as they 

 stain a greenish colour in the iodine green and Bismarck brown. 

 These are termed the Xylem or wood, and in longitudinal section 

 will be seen to consist of long fibres or tubes, marked by spiral or 

 other thickenings or pits on the walls. It has been shown conclu- 

 sively, by a series of elaborate experiments, that the function of 

 these tubes is to conduct water from the roots to the other parts of 

 th6 plant, and we can trace them to all parts of the plant, even to 

 the tips of the leaves and to the smaller branches. These groups 

 of tubes are called vascular bundles — i.e., bundles of vessels. 



Alternating with these bundles, and somewhat more circular or 

 perhaps tangentially oval in outline, are other bundles of small, 

 thin-walled cells (Fig. 2, ph.). These are not so clearly differen- 

 tiated from the surrounding tissues as the xylem bundles. They 

 have, however, a very definite structure and function. The struc- 

 ture can only be made out and the tissue clearly marked off from 

 the surrounding cells by longitudinal sections, in which it will be 

 seen that these bundles are made up principally of long tubes, 

 interrupted at intervals by a transverse wall, which is perforated 

 like a sieve. It is not easy for beginners to make out this struc- 

 ture in a root, as it requires extremely careful section-cutting, but 

 such sieve-tubes or sieve-vessels can be easily obtained by cutting 

 longitudinal sections of the stem of a cucurbitaceous plant. It 

 has been shown that this tissue, to which the name of phloem is 

 given, is concerned in the passage of food material down the plant 

 from the leaves through the stem to the roots. We have thus in 

 the central cylinder two distinct sets of conducting tubes : one for 



