2 4 



Commercial Gardening 



Fig. 5. Section across 

 Wood Cells, showing concen- 

 tric layers of woody matter 

 surrounding a central cavity 

 Scolopendrium 



Fig. 6. Elder Pith, 

 consisting of aggrega- 

 tions of Cells magni- 

 fied 



Changes in Cell Walls. Although the cells of the higher plants 

 may be all very much alike when they begin life, they vary immensely 

 in size, shape, and structure by the time they reach 

 full development, their ultimate construction being 

 dependent upon the functions they have to perform 

 for the wellbeing of the plant. The most common 

 change is the thickening of the cell wall internally, 

 by successive layers of cellulose, till 

 the internal cavity is nearly filled 

 up, and the cellulose gets converted 

 into wood (fig. 5). The cells of the 

 pith remain thin -walled (fig. 6). 

 Those on the outside of the trunk 

 of the Cork tree, Elm, Ash, &c., get 

 thickened like those of the wood; 

 but in this case the material is con- 

 verted into cork, which is very light 

 and almost impermeable by water. 



A thin layer on the outer face of all leathery leaves, like those of the India 

 Rubber and Palms, forms the cuticle, and is also of the nature of cork. 



Various Forms of Cells. With the exception of lowly plants like 



the duckweeds, the flowering plants gene- 

 rally furnish examples of cells of great 

 variety of form and length. Those which 

 become thread-like, but thickened inter- 

 nally, are termed wood cells (fig. 7), but 

 wood fibres when they become pointed at 

 the ends, with the thin portions spliced 

 or overlapping, so as to form continuous 

 masses of wood (fig. 7). The thickening 

 is by no means always uniform, for small 

 spots are left unthickened in pinewood, 

 and such are known as pitted wood cells 

 (fig. 7). The thickening may take the 

 form of single or double spiral bands in 

 the stems of Melons and Cucumbers (fig. 

 8), ring-like bands, or a mixture of an- 

 nular and spiral ones (fig. 8). In ferns 

 the bands unite in the form of a ladder. 

 These elongated cells may be placed end 

 to end and the partitions broken down, 

 thus forming continuous vessels, like a hose pipe, for the rapid convey- 

 ance of liquids. Sieve tubes are formed in the inner bark of stems by 

 the dividing plates of vessels becoming perforated by small openings. 

 Plants with a milky juice, like the Lettuce, Dandelion, and India Rubber } 

 have cells which join in a variety of ways and break down the inter- 



Tracheids 



Fibres 

 Fig. 7.-Wood Cells 



