OBSERVAT/ONS FOR YOUNG BOTANISTS 27 
parts of plants. Protoplasm, to live at all, must respire. Con- 
sequently if seeds be sown too deeply they become suffocated ; 
and similarly, if the roots of trees, &c., get into a wet, clayey 
soil, the oxygen of the air is impeded and they are liable to 
become unhealthy. Hence arises the importance of forking 
over the soil round the roots of plants when it has become 
hardened. 
Structure of Roots. — This will easily be seen to differ in 
some respects from stems, for without studying the anatomy, 
which requires the aid of a strong microscope, it may be 
noticed that the cortical layers are relatively thicker in roots, as 
also in subterranean stems, than in aerial branches. The use of 
this is to protect the central cord of wood, by which the chief 
strain is felt, when the aerial parts oscillate with the wind. In 
herbaceous plants the cortex frequently stores up starch and 
other nutritious foods for the plant’s future use. 
While the cortex is thick the fibrous layer (pericycle) is 
deficient ; for this is a tissue especially required by slender 
stems which easily bend to the wind but do not readily break. 
The pericycle of a root is employed in a different way, for it 
makes the thick fleshy outer part of our garden roots, as the 
parsnip, carrot, turnip, raddish, &c. 
Growth of Roots. — Now let us see how roots grow. If a 
seedling has acquired a good tap-root and then be allowed to 
grow vertically over water so as to be readily observed, and 
small dots be made with Indian ink from the apex backwards at 
intervals of half a line, i.e., one twenty-fourth part of an inch, 
after some hours it will be found that, while the terminal spot 
is carried onwards by the dead root-cap, a portion, about one 
line in length, i.e., containing two dots, has elongated, the dots 
behind having retained their original positions. If the piece 
thus lengthened be again marked, another small portion only 
will elongate as before. This shows that it is only a very 
small part of a root which elongates ; and this is obviously 
an advantage to the root, since it is obliged to insinuate itself 
among the resisting particles of soil. 
If, on the other hand, an internode of a stem of some rapidly 
growing plant, such as the hop, be marked, it will be found that 
all the dots will become more widely separated from each other 
after a definite time has elapsed. 
Aerial Roots. — Roots are not always underground, for 
many plants climb by means of modified roots, as in the familiar 
case of the ivy. In this plant the roots are formed on the 
shaded side of a shoot, and although in all probability originally 
stimulated into development by contact with a rock or tree, they 
are now often produced in anticipation of contact. Their use 
is to fix the ivy firmly to a wall ; but in tropical orchids the 
roots have an additional use, for they can absorb moisture, 
&c., from the surface of the tree upon which they grow. 
In the case of some fig trees, as the Indian Banyan, aerial 
