PLANT ORGANS AND ORGANISMS 
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Cotyledons. — Cotyledons are the first leaves to appear upon the 
ascending axis and are single in Monocotyledons, double in Dicoty- 
ledons. Occasionally, as in certain Maples, there may be three 
cotyledons shown. This is due to a splitting of one of the cotyledons. 
There exist no true cases of polycotyledony (development of many 
cotyledons) among Angiosperms as in Gymnosperms. In Mono- 
cotyledons the single cotyledon is a terminal structure and truly 
axial in relation to the hypocotyl and radicle. From a primitively 
Monocotyl-like ancestry Dicotyledons develop a second cotyledon 
on the Epicotyledonary node. Later, by a suppression of the second 
node the second cotyledon is brought to the level of the first. 
Scale Leaves. — Scale leaves are reduced foliage leaves. They are 
found on certain rhizomes, above ground stems, such as Dodder, 
etc., on bulbs, and forming the protective scales of scaly buds. 
Foliage Leaves. — These are the common green leaves so familiar 
to all. 
Bract leaves are modified leaves appearing on inflorescence axes. 
Sepals, petals, microsporophylls and megasporophylls are floral 
leaves and will be treated at length under the subject of the flower. 
Origin and Development of Leaves. — Leaves arise around the 
growing apex region of a stem or branch as lateral outgrowths, each 
consisting at first of a mass of cells called the primordial leaf. 
Through continued cell-division and differentiation of these cells in 
time the mature leaf is developed. The primordial leaf is formed 
by a portion of the dermatogen of the growing stem apex, which 
becomes epidermis, a portion of the periblem, producing mesophyll 
which grows into this, and a part of the plerome, which becomes 
vascular tissue within the mesophyll. 
In the sub-divisions of cells around the growing stem-apex, the 
primordial leaves (primordia) do not arise exactly at the same time. 
There is a tendency toward spiral arrangement. 
Phyllotaxy. — Phyllotaxy is the study of leaf arrangement upon 
the stem or branch, and this may be either alternate, opposite, 
whorled, or verticillate, or fascicled. It is a general law in the 
arrangement of leaves and of all other plant appendages that they 
are spirally disposed, or on a line which winds around the axis like 
the thread of a screw. The spiral line is formed by the union of 
