56 ELEMENTS OF BOTANY. 



77. Sieve-Tubes. — Grouped together with the bast fibres 

 of the stem there occur a peculiar and very important set oi 

 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 

 Wjith holes, forming a sieve-plate, like that shown in Pig. 44., 

 Sometimes the walls of sieve-tubes are more or less fully 

 covered with perforations, as shown in Kg. 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 v^ry generally, especially in 

 the expanded bases of the leaf- 

 stalks of leaves which move oi 

 their own accord and in siev& 

 cells, there is a direct connec- 

 FiG. 46. — Side View of Part of one tion of the Contents of one eel] 



of the Medullary Bays of Maple . , , , , „, , 



Wood. (Much magnified.) With another. The protoplasm, or 



semi-fluid layer with which all 

 active cells 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 

 lings (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 oi 



