156 PHARMACEUTICAL BOTANY 



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 



