1 8 PHARMACEUTICAL BOTANY 



and then smaller in calibre. These store starch and enable sap to pass 

 through. 



3. Endodermis, or innermost layer of cells in the cortex with len- 

 ticularly thickened radial walls. 



4. Pericambium, a zone of one, two, or three layers of rounded, 

 thin-walled, actively dividing cells, which may give rise to side rootlets. 



5. Radial fibrovascular bundle, which in most monocotyledons 

 consists of eight, twelve, or fifteen alternating patches of phloem with 

 radiating xylem arms between. Phloem tissue consists of phloem cells 

 and sieve tubes. Xylem at tips of arms, made of spiral tracheae the 

 first xylem elements to mature. Internal to these are small pitted 

 vessels, later, striking pitted vessels and considerable wood fibre. 



6. Pith. 



Dicotyledons. — ^The typical dicotyl root is a tetrarch one, four 

 xylem alternating with four phloem patches. These roots have an 

 unlimited power of growth. 



A. Of Primary Growth. 



A trans-section of a dicotyl root in its young growth shows the fol- 

 lowing structure from periphery toward centre. 



1. Epidermis with cutinized outer walls. 



2. Hypodermis. 



3. Cortex with usually small intercellular spaces. 



4. Endodermis, or innermost layer of cells of the cortex with radially 

 thickened walls. 



5. Pericambium of one to two layers of actively growing cells which 

 may produce side rootlets. 



6. Radial fibrovascular bundle of four, rarely two or three or five 

 or six phloem patches alternating with as many xylem arms. Not 

 uncommon to find bast or phloem fibre along outer face of each phloem 

 patch. Xylem has spiral tracheae, internal to these a few pitted vessels. 

 Then, as root ages, more pitted vessels, also xylem cells and wood 

 fibres make their appearance 



B. Of Secondary Growth. (Most ofiicial roots.) 



At about six weeks one notes cells dividing by tangential walls in 

 the inner curve of phloem patches. This is intrafascicular cambium. 

 A single layer of flattened cells starts to cut off on its inner side a quan- 

 tity of secondary xylem and pushes out the patches of bast fibres, adds 

 a little secondary phloem on the outer side. Secondary xylem finally 

 fills up the patches between the arms. The patches of bast fibres get 



