ABSORPTION IN PLANTS. 87 



lution, and the other an emulsioned, but not dissolved state Conditions of 

 of the food, so there are two absorbent systems, the lacteals 

 and the veins. The lacteals introduce such substances as are tion. 

 not absolutely dissolved, particularly the oils and fats. The veins ap- 

 pear to take up those substances only which are completely dissolved in 

 water. 



As in many other cases in physiology, so in this, a correct interpreta- 

 tion of the functions of the animal mechanism may be ob- Abgor tion in 

 tained by examining the corresponding structures and func-^ plants, their 

 tions in plants. In the more perfect of these, the absorption a 

 of watery material from the ground, constituting the ascending sap, is 

 brought about by the agency of collections of soft cells, which are placed 

 at the extremity of each rootlet. They are designated spongioles. By 

 their action the fluid is forced up through the tubes of the sap-wood into 

 the leaves, and there exposed to the conjoint agency of the sun and air. 

 A change is thus accomplished, and, from being crude, it turns into elab- 

 orated sap, and now descends through the bark, to be distributed to every 

 part of the plant. Its ascent is caused by the cells of the spongioles, its 

 descent by the chemical changes occurring among the cells which are 

 found in the structure of the leaves. 



These cells both of the roots and of the leaves are far from contin- 

 uing their action for an unlimited period of time. At the most, their 

 existence is transient. Those of the roots are gradually lost by decay, 

 or converted into solid structure, as the elongation of the organs through 

 the ground goes on. Those of the leaves are equally transitory. At 

 periodic intervals, both in deciduous and evergreen plants, the fall of the 

 leaf occurs a new organism succeeding in another summer to make up 

 for the one which has passed away. 



Whatever nutrient material is taken from the soil in the case of plants 

 is introduced by the aid of a cellular structure, and the cells die after ac- 

 complishing their duty. 



It was once a saying among physiologists that the lacteals are the 

 roots of animals, and in this there is, in reality, a great deal Analogy be- 

 of truth, for between the rootlet of a plant and the lacteal of 

 an animal there is a conspicuous relation, not only in struc- roots, 

 ture, but also in function. As is seen in Fig. 27, upon each villus of 

 the intestinal tube there is a layer of cylindric cells, underneath which the 

 lacteal vessel takes its rise, for it does not open by a free orifice on the 

 interior of the intestine, but its flask-shaped, loop-like, or convoluted or- 

 igin is obscurely seen in the midst of the cells. The aspect which the 

 villi present, from its doubtful nature, has led to the erroneous conclusion 

 that, as soon as active digestion goes forward, cells rapidly develop with- 

 in the epithelium, and continue to do so as long as the intestine contains 



