12 ELEMENTS OF PLANT ANATOMY. 
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intersections, or the tensions caused by growth are so strong as 
to cause a break across the entire wall, subsequent absorption 
removing the broken sections, till large spaces are formed 
where the secretion is stored away. The resin passages in the 
Coniferae, Taxus excepted, and in the root and stem of Hedera 
helix, also those containing volatile oil in roots of Umbelliferae 
and some Compositae, are examples of these organs. 
MILK TUBES. 
These occur only in certain families and differ from other 
organs of secretion in that they contain plastic material, as well 
FIG. 34. 
Network of milk tubes from fruit of poppy. x 150. —(Wiesner.) 
as the ordinary secretions which are not used in the construction 
of the parts of the plant. They contain no trace of protoplasm 
1 The first mode of development of intercellular spaces has been termed 
schizogenic, the second lysogenic. <A further distinction has been made between 
those intercellular spaces which are formed when the tissues begin to differ- 
entiate — protogenic — and those formed in older tissues — hysterogenic. 
