68 ELEMENTS OF BOTANY. 



The heartwood of a full-grown tree is hardly living, unless 

 some of the medullary rays may retain their vitality, and so 

 wood of this kind is useful to the tree mainly by the stiffness 

 which it gives to the trunk and larger branches, thus prevent- 

 ing them from being easily broken by storms. 



91. Movement of Water in the Stem. — The student has 

 already learned (§ 50) that large quantities of water are taken 

 up by the roots. 



Having become somewhat acquainted with the structure of 

 the stem, he is now in a position to investigate the question 

 how the various fluids, commonly known as sap, travel about 

 in it.^ It is important to notice that sap is by no means the 

 same substance everywhere and at all times. As it first makes 

 its way by osmotic action inward through the root-hairs of the 

 growing plant it differs but little from ordinary spring water 

 or well water. The liquid which flows from the cut stem of 

 a " bleeding " grapevine which has been pruned just before 

 the buds have begun to burst in the spring, is water with a. 

 little mucilaginous or slimy material added. The sap which 

 is obtained from maple trees in late winter or early spring, 

 and is boiled down for syrup or sugar, is still richer in 

 nutritious material than the water of the grapevine, while 

 the elaborated sap which is sent so abundantly into the ear of 

 corn, at its period of filling out, or into the growing pods of 

 beans and peas, or into the rapidly forming acorn or tho 

 chestnut, contain great stores of food, suited to sustain plant 

 or animal life. 



92. Experiment 18. Eise of Water in Exogenous Stems.— Cut 

 some short branches from a grapevine and stand the lower end of each 

 in red ink ; try the same experiment with twigs of oak, ash, or other 

 porous wood, and after some hours examine with the magnifying glass 

 and with the microscope, using the two-inch objective, successive cross- 

 sections of one or more twigs of each kind. Note exactly the portions 



1 See the paper on The so-called Sap of Trees and its Movements, by Prof. Chas 

 E, Barnes, Science, XXI, 535. 



