70 FIRST STUDIES IN PLANT LIFE 



enough to keep its place in the air. This thought 

 leads us to examine the blade to see how the leaf is 

 made strong. 



5. The leafs skeleton. Tear off from the stem the 

 leaf of a sow-thistle, and you will see that the stalk 

 gets its strength from several woody sap-tubes that 

 pass along it. These tough tubes run on into the 

 blade of the leaf, and are split up into a network of 

 veins that form the skeleton. Examine a skeleton 

 leaf — a leaf in which the soft parts have rotted away, 

 leaving only the hard parts. On this framework or 

 skeleton the soft parts of the leaf are spread. 



6. How the root-sap spreads over the leaf. Have 

 you ever seen a field that was being watered from a 

 main channel ? The water leaves the . main stream 

 for smaller channels, and from these runs into still 

 smaller channels, until the whole land is soaked. In 

 a similar way the root-sap flows from the large veins 

 to the small veins, and is thus spread all over the 

 leaf. We saw how the root sap-tubes run up through 

 the new wood into the leaf. This root-sap enters the 

 leaf along the upper part of the veins. Can you guess 

 from your peep inside of a leaf why the root-sap 

 spreads over the upper part of the leaf? "V\Tien the 

 smallest veins have been reached, the sap begins to 

 flow back through the under part of the veins. You 

 will notice that the veins stand out from the mider 

 surface of the leaf ; while they are sunk slightly into 

 the upper surface. If the edge of the leaf be torn, this 

 beautiful plan for the coming and going of the sap is 

 spoiled. Can this be the reason whj^ the leaf of a gum 

 tree has a vein that runs all round the leaf a little in 

 from the edge (fig. 123) ? Even when the edge is broken 



