THE ROOT. 17 
vascular cylinder, and composed of cells which differ from those 
of the growing-point chiefly in their elongated shape. Cambium 
and increase in thickness will be more fully spoken of on pp. 31 
and 36. 
We are now in a position to make a somewhat wider classifica- 
tion of tissues.» They may be grouped as follows :— 
I. Formative Tissue or Meristem. 
Composed of actively-dividing cells, with thin cellulose walls 
and abundant protoplasm. 
1. Primary Meristem. Making up growing-points and the 
whole of very young structures. 
2. Secondary Meristem or Cambium. Forming layers bounded 
internally and externally by permanent tissue. 
It. Permanent Tissue. 
Composed of cells not in a state of active division (though they 
may retain the power of dividing) and cell-derivates. 
1.1 Hpidermis. An external layer of flattened cells, to which 
hair-structures may be attached. | 
2. Ground-tissue. Largely composed of parenchyma, 
3. Vascular system. Composed of cells and vessels, 
1 See Note at end of Chapter. 
PHYSIOLOGY. 
The root of a land plant is protected to a great extent by its 
position in the soil, the root-caps preserving its delicate growing- 
points from injury. The epidermis at first protects from evapo- 
ration, and, in older examples, from which parts external to the 
vascular cylinder have peeled off, its place is taken by layers of 
cork (cf. p. 40). 
The root serves as an organ of support to the stem and leaves. 
It is often firmly fixed in the soil by numerous branches, and the 
nature of the vascular cylinder is such as to give firmness. The 
root is essentially a vegetative organ, 7.e., is concerned with main- 
taining the life of the individual, and consequently plays a very 
important part in Nutrition, since, in land plants, all the food, 
with the exception of carbon dioxide, is absorbed by it. The soil 
is made up of variously-sized particles, with spaces between them, 
that contain, according to circumstances, more or less water with 
various substances dissolved in it, the part not thus occupied 
being filled with air. Some of this water is readily drained off, 
and has been termed /ree water, while the remainder is in the 
form of films that surround the particles. These films consti- 
tute the hygroscopic water—the part which furnishes the plant 
with nutriment, and may be looked upon as a natural food 
solution. The absorption is effected by the young epidermice cells 
