ANATOMICAL STKUCTURE AND VITALITY. 67 



heating the stem and branches expands the sap, the buds 

 swell and the leaves unfold, the roots at the same time 

 perform their duty by producing young spo7igioles that 

 absorb fluid which by the vital action of the plant 

 is carried up through the cellular and vascular body of 

 the tree, enters the leaves (plant lungs), and through 

 the stomata comes in contact with and absorbs the 

 constituents of the atmosphere, forming fibre-sap. It 

 is then carried downwards through the vessels of the 

 inner bark, in its course depositing a soft matter called 

 cambium between the bark and wood of the tree which 

 solidifies and becomes new wood, thus marking the yearly 

 growth of exogenous trees. The bark also assimilates 

 its constituent elements, according to the nature of the 

 plant. In many plants, such as the family to which the 

 Marvel of Peru and evening primrose belong, the roots 

 of orchids, succulent mesembryanthemums, and stalks of 

 rhubarb, bundles of needle-like bodies are dispersed 

 throughout the cellular structure called raphides ; in 

 some species of cactus they are extremely abundant, and 

 in form like grains of sand, which are insoluble in water. 

 They consist chiefly of phosphate of lime, but their use 

 in the life of the plant is not well ascertained. That the 

 sap has a general periodic ascent and descent is manifest, 

 but the microscope shows that a local circulation goes on 

 independent of the movement of the sap upwards and 

 downwards. One kind especially observed in milky 

 plants, is in vessels difi'ering from the usual tubular ones 

 in being branched and anastomosing to each other ; they 

 are called laticiferous vessels, and the milky granules 

 seen circulating, latex. This being difiicult to observe, 

 it is not known if it is common to plants in general, but 

 by the aid of a powerful microscope it may be seen in the 



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