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salts of lime, potass, magnesia, &c., and thus the fall of the 

 leaf is not an useless process in the economy of nature, but a 

 means of restoring again to the earth large quantities of 

 material for the nutriment of plants in the following season. 



The fluids absorbed by the roots are quickly diflused 

 through the plant and carried up to the leaves, which by 

 exhalation constantly require a fresh supply, and draw it 

 upward to replace the loss, thus promoting a general move- 

 ment through every part of the plant. In cellular plants 

 the circulation is general throughout the tissues ; in ferns 

 the fluid absorbed passes along the loose cellular tissue around 

 the scalariform vessels ; in Monocotyledons it a scends through 

 the elongated cells surrounding the spiral vessels, which 

 ordinarily contain air ; in Dicotyledons the spring sap flows 

 gradually up the stem, at first filling the spiral vessels, but 

 as the leaves expand they exhale fluid, and then the spiral 

 vessels contain air, and the sap passes by the newer wood 

 vessels and cells ; and in summer we have the elaborated 

 sap sent back from the leaves in an external current between 

 the bark and the wood, from whence it is diflused through 

 all the active cells and vessels of the stem. 



The first true insight into the reproductive process in 

 plants appears to have been made known in this country by 

 Grew in a paper read before the Royal Society in 1676, and 

 next adopted by Ray in 1694. LinnsDus in 1736 gave to the 

 world his celebrated sexual system, founded on the repro- 

 ductive organs ; but although it was known that pollen was 

 necessary for impregnation, its modus operandi was not under- 

 stood, until in 1823 Amici discovered the pollen tubes, and 

 Robert Brown traced them to the nucleus of the ovule. 



To the important part played by insects in the fertilization 

 of plants I need not here refer, nor to the various modifica- 

 tions in the structure of the style or stigma, designed to 

 fulfil special ends ; but it may be of interest to place before 



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