Practical Plant Biology. 



manner as the activity of the cambium of the stem adds to the 

 thickness of the wood in that organ. 



The arrangement and growth of the tissues outside the conduct- 

 ing tracts is much the same in the root as in the stem, except 

 that near the tips the outermost layer of all contains certain cells 

 which elongate themselves at right angles to the surface and 

 grow out as tubular cells into the surrounding soil. These are the 

 root-hairs. 



The tip of the root is composed of growing cells like those of 

 the extreme apex of the stem, and from them the cells of the 

 cortex and the cells of the procambial core are differentiated. At 

 the same time cells are shed off on the outside of this growing 

 tip and form the root-cap, which is a mass of loose cells through 

 which the tip forces its way. The cells which are shed off lubricate 

 the passage of the tip through the soil. 



From this short description of the origin, growth and arrange- 

 ment of tissues you will understand that the water and dissolved 

 substances passing into the root-hairs are transmitted by the cortex 

 to the primary wood and to the protoxylem in the. ends of the 

 roots. These are continuous with the outer and more recent 

 secondary wood next to the cambium and pass the water collected 

 by the root-hairs up to the most recent parts of the wood of the 

 stem. These outer layers of the wood of the stem are continuous 

 with the primary wood and protoxylem of the newly formed branch- 

 tips, which are in direct connection with the woody strands coming 

 from the leaves. Hence even in the oldest trees there is a direct 

 channel in the recently formed wood connecting the absorbing 

 tips of the roots with the evaporating surfaces of the leaves. 



PRACTICAL WORK. 



Take the end of a branch of Finns silvestris. Identify the long and dwarf 

 shoots. Observe the circle of long shoots formed round the main stem at the 

 base of each year's growth. Notice the subtending scale of the dwarf shoots, 

 and their sheathing scales surrounding the bases of the two foliage-leaves. 

 Make out the number of seasons during which the dwarf shoots persist. Re- 

 cord these observations in your sketches. 



Prepare and mount thin transverse sections of a leaf. Make a sketch 

 showing the epidermis with its thick walls and its cuticle, the sunken stomata, 

 the inner fibrous layer, the mesophyll with its inflected cell-walls, the bifurcating 

 cells beneath the stomata, the resin-ducts, the endodermis, the transfusion 

 tissue, and the two conducting tracts formed of wood and bast. 



Prepare and mount a transverse section of the terminal soft and succulent 

 part of a branch, cut in early summer before the foliage leaves on the dwarf 

 shoots are mature. Sketch the section marking the epidermis, cortex, primary 

 medullary rays and pith. The primary vascular bundles composed of protoxy- 



