74 Plants and their Ways in South Africa 



bundles serve for the vertical distribution of food and food 

 material. Starch is also stored here. 



The cells outside of the bundles and within the endodermis 

 comprise the pericycle. This consists of several layers of 

 cells ; those just outside the phloem being developed into thick- 

 walled fibres serving as a protection to the delicate cells of 

 this tissue. The fibres stain with iodine nearly the same colour 

 as the wood. They are sometimes called phloem- or bast- 

 fibres, but they are not developed from the same region as the 

 bundles. 



Between phloem and xylem the delicate cambium cells 

 are found. Some distance below the tip of the stem, the cells 

 of the medullary ray become active and by their division form 

 cambium (interfascicular cambium) which unites with the cam- 

 bium of the bundle and forms a complete cambium ring around 

 the stem. This ring, by the constant division of its cells, increases 

 in breadth and circumference ; some of its cells, those toward 

 the pith, form new wood; toward the outside phloem is also 

 renewed by them (secondary xylem and phloem) ; the medul- 

 lary rays are also extended by the cambium. 



Within the cortex and pith are small dots which indicate 

 the position of resin cavities. Where growth is vigorous resin 

 cavities are also found in the secondary phloem. 



As the Sunflower is an annual, but little secondary wood 

 and phloem are formed, and correlated with this fact, little if 

 any cork is developed. 



Bundles with cambium are known as open as contrasted 

 with those with no cambium which are closed, that is, closed 

 to further development. 



The bundles of the Sunflower which have the characteristic 

 structure of dicotyledons are of the type known as collateral ; 

 they have the phloem lying to one side of the xylem and in 

 the same radius. In the Pumpkin family there is phloem to 

 the inside as well as the outside of the bundle, a bi-CoUateral 

 type. The phloem may surround the xylem ; the bundles are 

 then known as concentric. 



Before proceeding farther it may be well to define a term 

 which has been frequently used in this chapter, the word tissue. 



