208 



THE SECOND BOOK OF BOTANY. 



FIG. 378. 



tuce, dandelion, etc., there exudes a milky sap, called 

 latex. There is a peculiar system of vessels containing 

 this milky juice, known as laticiferous vessels. They 

 form an irregular net-work, as 

 seen in Fig. 378. It is thought 

 that these so-called laticiferous 

 vessels are not true vessels made 

 up of cells placed end to end, 

 but only intercellular spaces 

 with walls formed by a deposit 

 from the fluid that fills them. 

 They never have markings up- 

 on their walls like the vessels 

 we have been studying. When 

 young, laticiferous vessels are 

 extremely small, averaging less 

 than -^Vfr of an inch in diame- 

 ter, and can only be seen under 



high magnifying powers. Old vessels, when swollen 

 from accumulations of their milky sap, are more ap- 

 parent. As you see by Fig. 378, they are cylindrical, 

 and the branches are as large as the veins, forming a 

 sort of net-work. But this kind of vegetable struct- 

 ure is not well understood. 



EXERCISE LXV. 

 The Contents of Cells. 



The contents of cells vary with their stage of 

 growth. When very young they usually contain only 

 the nucleus and protoplasm, but, as they approach 



