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 ot 
diffuse laterally, but not flow. When the cells become 
mature and cease growth, the pits often become pores by 
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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 climates, 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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