TISSUES AND SIMPLE ORGANS. 



77 



sion) rarely or never (?) occurs in the simple milk-tubes. There is 

 nothing definitely known concerning the physiology and movement 

 of the fluid within the laticiferous vessels. The following may, 

 however, suffice to give an idea of the probable condition of affairs : 

 The milky fluid is composed mainly of water, fats, starch, tannin, 

 and grains of resin ; it is usually white, more rarely yellow or red. 

 Observations, even those of a purely comparative character, indi- 

 cate that the milk-tubes obtain at least their carbohydrates from 



FIG. 45. Cross-section of the stem of Selaginella incequalifolia. 

 (x!50.) (After Sachs.) 



the centres of assimilation, namely, from the palisade- cells of the 

 leaves. The fact that the milk-sap sometimes becomes watery, for 

 example, when seeds germinate in the dark, would seem to indi- 

 cate that it is a formative substance. In certain cases it has been 

 observed that the presence of milk-tubes in leaves corresponded to 

 a diminution of the conducting-parenchyma of the vascular bundle. 

 In reference to the movements of the milk-sap we must make a 

 distinction between thick-walled, and thin-walled milk-tubes. In 

 the latter it is essentially the hydrostatic pressure of the surround- 



