ON THE STRUCTURE OF THE ROOT. 75 



vessels are formed, strengthened by spiral, reticular, and irregular 

 thickening of the wall ; while at the same time the protoplasm 

 disappears, so that these cells may now be said to be dead. 



Modifications of a similar nature take place in the production 

 of phloem, but here the transverse walls are only partially broken 

 down and converted into sieves, through which insoluble proteid 

 matter, which is not capable of passing through organic mem- 

 branes, is able to make its way. It must not be supposed that the 

 sieve-plate functions as a sieve in the sense of sifting one thing 

 from another. The reason for the existence of sieve-plates in 

 what would be otherwise an open tube may be partly due to the 

 necessity for some strengthening partition to prevent the collapse 

 of such long, thin-walled tubes, and partly to enable the tubes to 

 be completely closed at certain times — notably in winter, when 

 very little active formation of food materials is taking place. 



The parenchymatous cells of the cortex and the central cylin- 

 der are only slightly altered in structure from those described in 

 Fig. 6. They possess a thin layer of protoplasm on their walls, a 

 large central vacuole containing cell-sap, and a nucleus generally 

 found in the layer of protoplasm on the cell-wall (Fig. 4). 



Such is the general structure of the root, and the roots of all 

 flowering plants will be found to possess this structure in principle 

 at an early stage in their development. At a later stage, in the 

 root of the Lime and others of a Uke nature, secondary formations 

 appear, resulting in the production of a much larger quantity of 

 xylem, until the greater part of the root becomes woody. This is 

 the condition of all the older roots of our perennial dicotyledons. 



EXPLANATION OF PLATE IV. 



Fig. 1. — Transverse section of root, about 1 in. behind the apex, x 4 : 



c, Cortex; c.c. Central cylinder. 



,, 2. — The same section, magnified about 150 diameters : — 



2).l., Remains of the piliferous layer; pt., Outermost cortical 

 layer, protective layer; c. Cortex; e., Endodermis ; p.c. , 

 pericycle ; /., Fundamental tissue^parenchyma ; 2^h., Phloem 

 or bast ; x., Xylem or wood ; j}., Pith. 



,, 3. — Section showing piliferous layer (somewhat diagrammatic) : — 

 l^.l., Piliferous layer; r.h., Root-hairs; pt, Outermost layer 

 of cortex. 



