22 ELEMENTARY ORGANS OP PLANTS. 



mcning tubes of the latex is ultimately developed. The 

 secretion of the latex may be readily seen in dandelion, 

 euphorbia, or celandine, by breaking any leaf or shoot of 

 these plants. 



15. Besides the intercellular spaces and passages already 

 described, other cavities exist in the parenchyma or cellular 

 tissue, which are usually filled with air, and are called lacunae 

 or air-cells. In aquatic plants large lacunae or air-cells are 

 formed in the parenchyma of the stem and leaves, by the 

 rupture or absorption of the septa between a number of con- 

 tiguous cells. These laounse, in the pond-weeds, assume a 

 regular form, which is constant in each species ; their forma- 

 tion, therefore, cannot be attributed to accident ; on the con- 

 trary, they seem to be organically necessary to the healthy 

 exercise of the functions of these plants, giving them the 

 requisite degree of buoyancy in the water. 



16. More frequently, however, the lacunae are very irregu- 

 lar in their form, and are produced by the rapid growth of 

 the parts resulting in the irregular destruction or distension 

 of the cellular tissue. In these instances, their formation is 

 in some sort accidental. The hollow stems of grasses and of 

 the umbelliferae are produced in this way. 



17. After the first layer of protoplasm has been formed 

 from the cambium and the cells constructed, as growth pro- 

 gresses, the cells, at first extensible and compressible, ulti- 

 mately assume fixed and unyielding forms, owing to the 

 internal deposition on their walls of sclerogen. The de- 

 posited matter includes all the various substances introduced 

 into the circulatory system of the plant, and left by evapora- 



