THE VEGETATIVE ORGANS OF PLANTS. 



273 



sorption and removal an uninterrupted tube has been 

 formed. In these large dotted ducts there appears to be 

 no direct communication with the surrounding cells 

 through their sides. The dots or pits are simply very thin, 

 points in the cell-wall, through which sap may soak or 

 diffuse laterally, but not flow. When the cells become 

 mature and cease growth, the pits often become fiores by 



Fig. 50. 



absorption of the membrane, so that the ducts thus enter 

 into direct communication with each other. 



Exogenous plants are those whose stems continually 

 enlarge in diameter by the formation of new tissue near 

 the outside of the stem. They are outside-growers. Their 

 seeds are usually made up of two loosely united parts, or 

 cotyledons, wherefore they are designated dicotyledonous. 

 All the forest trees of temperate cUmates, and, among 

 agricultural plants, the bean, pea, clover, potato, beet, tur- 

 nip, flax, etc., are exogens. 



In the exogenous stem the bundles of ducts and fibers 

 that appear in the cell-tissue are always formed just within 

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