56 ELEMENTS OF BOTANY. 
77. Sieve-Tubes. — Grouped together with the bast fibres 
of the stem there occur a peculiar and very important set of 
vessels called sieve-tubes. The student cannot easily make 
these out from sections of ordinary stems, but it is not diffi- 
cult to understand their structure in a general way. These 
tubes arise from the partial union of large cells which stand 
in rows, united end to end, as shown in Figs. 42, 43. The 
partitions between adjacent cells gradually become perforated 
with holes, forming a sieve-plate, like that shown in Fig. 44. 
Sometimes the walls of sieve-tubes are more or less fully 
covered with perforations, as shown in Fig. 45. 
Continuity of the Living Cell-Contents.— It was formerly 
supposed that cells of plants were entirely shut off from 
each other while living. Recently, 
careful investigations have shown 
that very generally, especially in 
the expanded bases of the leaf- 
stalks of leaves which move of 
their own accord and in sieve: 
cells, there is a direct connec- 
Fic. 46.— Side View of Part ofone tion of the contents of one cell] 
ey Pee csctiea Maple with another. The protoplasm, ox 
semi-fluid layer with which all 
active celis are lined, and in which their life and working- 
power resides (Chapter XIII), extends in delicate threads 
through the cell walls, and connects in all directions with the 
protoplasm of other cells. 
78. Longitudinal Section of the Stem. —'The knowledge of 
stem-structure that can be gained from a longitudinal section . 
of any kind of wood depends upon the way in which the sec- 
tion is cut; that is, whether it is at right angles to the annual 
rings (radial section), or parallel to the rings (tangential sec- 
tion). The wood-cells, of which the student has in the cross- 
section seen only the cut-off ends, appearing as circular or 
