36 FIRST STUDIES IN PLANT LIFE 



there are so many. It must be a stout stem that can 

 carry all these leaves. And each year there are more 

 leaves, and the stem must become stronger. And so 

 the stem gets thicker and thicker as the leaves grow 

 more and more, until you cannot put your arms round 

 the stem. And, as the tree grows older, all the middle 

 part of the stem — the heartwood — becomes dead and 

 hard. It is this hard, dead heartwood that enables 

 the tree to hold up its great mass of leaves and to 

 face the storm. Look at a great gum tree in a strong 

 wind. The young saplings near it that have as yet no 

 heartwood bend almost to the ground ; but the old 

 gum tree bends not at all. Not for a thousand years 

 has the old gum tree bent to the wind. 



4. Where the new stem-wood is placed. — Can 

 you guess where the tree puts the new wood it makes 

 during the growing season ? In the middle ? No : 

 we have seen that the middle becomes dead and hard. 

 Where then ? On the outside of the old wood, and just 

 below the bark. The living, active wood in a tree is 

 to be found close to the bark. 



5. The bark. — And the bark — does it also put the 

 new bark on the outside ? No : you can see that the 

 bark must put its new growth on the inside, because, 

 the outside gets old and cracked, and often, indeed, 

 splits or peels off.* The bark is sometimes very 

 elastic, and is able to stretch without breaking as the 

 stem grows, but in most trees it splits or peels. In 

 the elm it breaks into ridges, and in the stringy- bark 

 gum it peels off. The outer bark may be very thick, 

 as in the cork oak or the stringy-bark gum, or it may 



*The thin formative layer which, on one side, makes the new wood, and 

 on the other side, makes the new bark, is called the cambium {cnmbio^ I 

 change). 



