46 PLANT LIFE. 



glands. The hollow stems of grasses, hemlock, parsnip, also 

 the hollow leaves of the onion, are formed by the present 

 method. Schizogenous intercellular spaces often contain 

 air, which serves in connection with food or respiration. In 

 aquatic plants, the presence of air, in addition to the above 

 functions, also enables the plants to float, and for this 

 purpose the cavities are frequently very large and highly 

 specialized ; the large bladders of the common brown 

 seaweed called " bladder-wrack " being entirely for this 

 purpose. Amongst other contents may be mentioned 

 resin, or balsam, a solution of resin in an ethereal oil ; in fir 

 trees and their allies, oils and gum-resins. 



Cell-fusions take place when the partition walls of adjoining 

 cells become absorbed, the large cavity formed being still 

 bounded by the walls of the component cells. In this way 

 true vessels are formed, by the absorption of certain of the 

 transverse walls of superposed cells. Laticiferous vessels 

 consist of straight or branched and often anastomosing rows 

 of cells running in many cases through various tissues of 

 the plant, having the transverse walls entirely or partially 

 absorbed, and containing a milky juice called latex which 

 escapes in considerable quantities when the plant is cut, as 

 seen in the flower-stalk of the dandelion. Amongst economic 

 products furnished by latex may be mentioned opium and 

 india-rubber. The exact use of latex in plant economy is not 

 known. Under the same heading may be placed sieve-tubes, 

 having the greatly thickened transverse walls perforated by 

 numerous small openings ; these perforated transverse plates 

 are called sieve-plates. Not unfrequently certain contiguous 

 portions of the lateral walls of two sieve-tubes become 

 similarly thickened and perforated. The object of this 

 perforation of the walls is to facilitate the circulation of 



