74 ELEMENTS OF PLANT ANATOMY. 
cross-walls, so as to form continuous tubes. Examples of this 
class are found in Cichoraceae, Papaveraceae, and Papayaceae. 
The non-articulated arise from a single cell developing into a 
long branched sac whose branches grow at their apices and push 
their way between other tissues. Such tubes are found in the 
families Euphorbiaceae, Urticaceae, Apocynaceae, and others. 
They may be called also milk-cells in distinction from the 
former, which are really ducts. 
Milk tubes are said to originate early in the young embryo. 
Schmalhausen says that in Euphorbia they start from a cell 
outside the plerome near the base of the cotyledon. ‘This cell 
grows, branches, and extends through the entire plant, which 
sometimes reaches the height of a man. Others deny this and 
claim that the tubes originate from separate cells at the nodes 
of these plants. The general course which they take is such as 
to indicate a function similar to that of the sieve-tubes, but 
little is definitely known in this regard. 
SUMMARY. 
The foregoing description includes the principal kinds of 
cells and cell derivatives, as the ducts are sometimes called. 
The first division into tissues is that of meristem and lasting 
tissue. Of meristem there are three kinds: first, urmeristem, 
or the little cluster of cells lying at the extremity of the axis, 
which we call initial cells; second, primary meristem, or the 
dermatogen, periblem, and plerome ; third, secondary meristem, 
which includes all meristem cells which were once in the condi- 
tion of lasting tissue. The lasting tissues are classed in three 
systems, epidermal, vascular, and ground systems. In addition 
to these systems there are certain classes of cells forming organs 
for special functions, which organs are called secretion holders. 
