296 
Supplement to the "Tropical Agriculturist" [Oct. 1, 1901 
gent study of nature necessitates observation, and 
provides a mental discipline that cannot fail to 
develop those powers of the intellect which it is 
the object of true education to discover, to 
cultivate, and to strengthen, (fjoiid cheer?.) 
NATURE STUDY LESSONS. 
SECOND COURSE IN THE STUDY OF PLANT LIFE. 
THE STEM. 
From the position of the stem it would strike 
you that it is a means of communication between 
the roots and tlie leaves of plants (tlie branches 
large and small being division? and subdivisions 
of the stem). But — 'jrou might well ask — what 
is the nature of this communication, und how 
is it carried on? Before answering this question, 
however, I must tell you something about the 
structure of tlie stem. Now I do not mean to 
explain to you its ultimate or minute structure, 
for if I tried to do that 1 would probably 
perplex you, and that is just what I want to 
nvoid doing. The study of minute structure 
in plants (or animals) is a special study known 
as Histology, a knowledge of which is not 
necessary for you to understand the functions 
of the stem. Indeed, I do not want you to 
acquire a special knowledge of Botany, but to 
know enough of the plant and the functions 
of its various parts or organs to enable you to 
understand its wants so as to be in a 
position to supply those wants. 
The stem — and in fact every part of a plant — 
is made up of minute cells wliich differ j,i 
size, shape and other characters, and these cells 
combine in various ways to form what are 
called tissues or cell masses, which of course 
differ according to the character of the cells 
that go to form them. Tliere are also asso- 
ciated with tissues, vessels which you might 
think of as minute tubes each formed out 
of two or more cells. 
The structure of the stem is characterised 
by a more or less definite arrangement. Let us 
take the stem of the mango as representing 
a very large class of plants. If you examine 
the stem of the mango, cut right across, you 
will be able to distinguish the bark on the 
outside, next to it the tvood occupying much 
the larger space,; and, generally, in the centre, 
the pith. Buo between the bark and the wood 
is a very thin but very important hfver of 
cells forming wliat is known as the caynbium 
or growing layer, which has the power of 
forming wood on the one sid« and bark on 
the othe., and so enabling the tree to grow 
ia ' thickness. Similar active growing tissue 
occurs at the ends or growing points of stems 
and branches enabling them to develope in length 
or height. 
See if you could distinguish in a cross 
section of a mango or rose stem a number of 
radiating lines running from the pitli to the 
cambium. Tliese are the medullary rays which 
serv« as a means of cotnumt)iC(it>ioa b^tweea 
(b9 yarious j»arts, 
The stem of the cico:iut as representing another 
large class of plants is r.ither diff^irent in 
structure. Here the tissues do not go to form 
successive layers of bark, cambium, wood and 
pith, bur, when examined on the cross cut, 
shows a ground work of soft tissue through 
which the harder potions are irregularly scattered. 
The so-called bark is really cf the same 
character, but contains a very much larger 
proportion of the hard tissue. 
It is the hard or woody parts of a stem 
tint perform the function of cotiveying fluids 
(and all plant food must be in ths fluid 
condition as I will explain later on) from the 
roots to the leave'. You have all, no doubt, 
heard the expression "sap-wood," and can 
probably distinguish the sap-wood from the 
"heart-wood," Well, it is the sap-wood or fresh 
outer wood that carries on the work of con- 
veying fluid matter upward-. The inner harder 
dried wood is what affords us timber. 
You must know that there is a good deal 
of water passing out in the form of vapour 
through the leaves. (I will tell you more about 
this when we come to speak of leaves.) Now 
the constant evaporation from the leaves tends 
to bring about an upward current of fluid 
through the stem. This general explanation is, 
I think, sufficient for you to understand ono 
of the important functions performed by the 
stem, viz., the conveying upwards of liquid 
plant food from the roots to the leaves. 
Now, I have no doubt, you are ready to 
put me another question, and that is— what 
happens to the food materials that are con- 
veyed to the leaves? To fully explain this 
I shall have to tell you a great deal about 
the functions of leave?, concerning which I 
have yet to speak to you. But this much 
I will tell yow now — the plant food taken up 
by the roots and passed up by the stem is 
prepared, or made lit for the use of the plant 
in the leaves. 
So far as we know, therefore, the courie 
which the ascending sap, as it is called, takes 
is through the cells forming the younger laj'ers 
of wood, the vessels assisting whe:i the current 
is rapid ; while the prepared sap is generally 
believed to descend and circulate through the 
inner layers of the bark. [The baik I should 
tell you consists really of three layers which 
can be well seen in the "shoe-flower" stem 
as (1) an outer "skinny ' layer, (2) a middle 
greenish layer, and (3) an inner fibrous layer.] 
If the above facts be true, then if you remove 
a ring of bark from the stem of, say a mango 
or orange tree, or bind the stem very tightly 
with a strong hoop, no increase of wood should 
take place below the ring or hoop, while a 
thickening should occur immediately above it, — 
for the reason that while the unpreparel fluid 
sap travels upward through the wood celfs, 
the prepared sap cannot djscend below a cer- 
tain point where active growth results from 
excessive nutriment. This is an experimenfi 
worth trying, and you should try it. 
I have already told you that certain uuder- 
g;rout)d stems become ^woUoq tm4 store 
