196 PHARMACEUTICAL BOTANY 
5. PRIMARY FIBRO-VASCULAR BuNDLEs of open collateral type 
arranged in a circle with primary medullary rays between the 
bundles. Each bundle shows an outer protophloem, a cambium 
called intrafascicular cambium, because it is within the bundle, and 
an inner protoxylem. . 
6. Piru, composed of parenchyma cells. 
If a series of additional cross sections are now cut at different 
descending levels along the Sunflower stem until the old part of 
Mechanical Tissue, Phloem 
Parench | mbium 
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Fic. 126.—Portion of a longitudinal section of a sunflower (Helianthus) stem. 
(From “A Text Book of General Botany’? by Smith, Overton et al. The Macmillan Co. 
Publishers.) 
the stem near the ground is reached, and these be examined 
microscopically, it will be noted that some secondary changes 
have taken place which explain the gradual increase in thickness 
of this stem as the base is approached. One of the first of these 
changes incident to secondary growth is that observed in the 
cells of the primary medullary rays between the intrafascicular 
camlia of the various bundles. These lay down tangential walls 
to form interfascicular cambia or cambiums between the bundles. 
When this process is completed, a continuous cambium ring is 
formed. 
