INTRODUCTION 
A tree is a woody plant which produces one main central up- 
right axis or stem that does not branch for some distance above 
the ground. <A shrub is also a woody plant, but its stem always 
branches directly at the ground. Some plants are either trees or 
shrubs, as, for example, the well known mesquite or the willows. 
The tree, as a whole, is an active, living, working being, en- 
dowed with all the vital activities peculiar to plants, and sensi- 
tive to the physical and biological environment in which it lives. 
A tree is composed of stem or trunk, roots, leaves, branches, buds, 
flowers, fruit and seed. 
The leaf is an expanded, usually flattened branch of limited 
erowth, consisting of two main parts: the blade or expanded 
portion, and the petiole or stalk which connects the blade with 
the shoot. A pair of leaf-like appendages, knows as stipules, are 
sometimes attached to the petiole. There is a wide variety in 
shape, size, form and structure of leaves, and it is on these char- 
acteristics that the key for the separation of the species is largely 
based. 
The leaf performs three very important functions, namely: 
photosynthesis, transpiration and respiration. 
The word, photosynthesis, means synthesis by means of light. 
In this process, the green, expanded portion of the leaf, in the 
presence of sunlight, unites carbon dioxide, which it receives 
from the air, and water, which it receives from the soil, forming a 
new compound, carbohydrate, and liberating oxygen, which es- 
capes from the leaf into the atmosphere as a waste product. 
Thus the green leaf consumes a gas which cannot support com- 
bustion or be used in respiration by animals or plants, and re- 
leases one which ean be so utilized. This explains the purifying 
effect of green plants on the atmosphere. This process is entirely 
dependent upon sunlight, and will not take place unless the 
leaf is properly illuminated. It does not occur any place in 
nature except in green tissue of plants, and a similar union of 
the crude materials, carbon dioxide and water to form sugar, 
has not been brought about, so far, by our most ingenious 
chemists. 
1—Trees. 
