56 



ELEMENTS OE 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 caunot 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.' Eecently, 

 careful investigations have shown 

 that very generally, especially in 

 the expanded bases of the leaf- 

 stalks of leaves which move ol 

 their own accord and in siev© 

 cells, there is a direct connec- 

 tion of the contents of one eel] 

 with another. The, protoplasin, 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 th(* 

 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-ofE ends, appearing as circular or 



Fig. 46. — Side View of Part of one 

 of the Medullary Rays of Maple 

 Wood. (Miicli magnified.) 



