TISSUES AND SIMPLE ORGANS. 



77 



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

 nothing definitely know-n concerning the pliysiology and movement 

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

 however, suflice 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. 

 (xl50.) (After Sachs.) 



the centres of assimilation, namely, from the jDalisade- 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 corres2:>onded to 

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

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

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

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



