The Orchard Plant 59 



allow some of the stored food in the stem to enter these 

 water passages in the spring of the year, as at this time the 

 buds are swelling, and an extra food supply is needed at 

 once in these parts. 



Transformed formative tissue extends outward be- 

 tween the bundles of water-carrying vessels, as shown at 

 3. As the bundles increase in thickness and in numbers, 

 they so crowd these cells that they are contracted into 

 very small space. A cross-section of any limb or trunk 

 shows these flattened cells as fine white lines that radiate 

 outward from the center. These are called medullary 

 rays, and their function is to allow a movement of both 

 food and water to and from the inner, living tissues. 



Other sets of vessels or tubes are shown at 4 on the 

 inner side of the bark. These are not so large as the water 

 vessels, neither are they so nearly continuous in their 

 connection. These are known as sieve tubes, and their 

 function is to carry food that has been elaborated in the 

 leaves, in either direction, wherever the demands of growth 

 require. 



The cambium or growing layer is shown at 5. This 

 is composed of a narrow zone of small cells, out of which 

 the water vessels and eventually wood are formed on the 

 inner side and sieve-tubes and the several layers that com- 

 pose the bark on the outer side. This is the only region in 

 the entire stem or trunk in which growth is possible in 

 orchard plants. 



The bark does not ordinarily increase in thickness to 

 any great extent. The outer layers serve as a protection 

 to the delicate tissues within. As a new layer is formed 

 each season from the outer cambium layer, the outer layers 



