AN INTRODUCTION TO PLANT BIOLOGY 95 



in the woody part. Cut a thin section and note that the red color 

 is in the wood-tubes of the nbro-vascular bundles which were men- 

 tioned in 70. 



If some stems be left in the ink for several hours, the sections will 

 show that the red ink soaks (osmoses) from the wood-tubes into the 

 bark and pith. 



From such experiments as those above we conclude that 

 the path of water up the stem is through the nbro-vascular 

 bundles in the woody part of the stem. It was through 

 these tubes that the water passed in our experiment on root- 

 pressure (see 86) and in that on evaporation ( 91). From 

 the wood-tubes water slowly osmoses into the cells of the 

 bark and pith. 



93. Water in the Leaf, and Evaporation. In the last 

 experiment the red-colored water ascended the stem and 

 passed along the veins of the leaf. This was possible be- 

 cause the veins of the leaf are bundles of tubes directly 

 connected with the wood-tubes in the stem. 



(D) Take a leaf from the bean or other shoot which was used in 

 92. Scrape the petiole and veins so as to show the bundles of 

 tubes colored by the red ink. Note (especially good in the bean 

 and in celery) the connection of the bundles of tubes in the veins of 

 the leaf with the bundles of the stem ; this is easily done by care- 

 fully scraping away the surface tissue from one side of the stem and 

 petiole until the colored bundles of tubes are uncovered. 



It is evident from the arrangement of the tubes and the 

 path taken by the ink that water can pass directly from the 

 wood-tubes of the stem into those of the veins of the leaf, and 

 thence into the numerous veinlets. In this way water 

 coming up the stem from the root is distributed throughout 

 the leaf, which is thin and greatly expanded so as to expose 

 as much surface as possible to sunlight and air, and thus pro- 

 mote rapid evaporation. It is obvious that one purpose of 

 the veins of the leaf is to distribute or spread the water so 

 that it may be evaporated rapidly. That the arrangement is 

 very efficient is indicated by. the fact that the leaves of a 



