state; pomoi,ogical society. 123 



strengthened by a spiral arrangement which reminds one of 

 the metal reinforcement used on lines of rubber hose de- 

 signed to withstand great pressure, only the spirals are on the 

 inside rather than on the outside of the tube. It must not be 

 assumed that the tissues of trees are not subjected to consider- 

 able pressure from this upward flow of sap. On a steam gauge 

 attached to a maple tree I have obtained a pressure amounting 

 to twenty-five pounds to the square inch. 



Running radially out from the center of the trunk are rows 

 of short cells whose shape and arrangement may be likened to 

 a brick wall. These cells remain alive much longer than many 

 of the woody cells which adjoin them, and they serve to unite 

 all the separate living tissues of the stem. They are active in 

 transporting food materials from the outside inward and are 

 connected with the water conducting elements an:l serve as 

 storehouses for starch, etc. They form the so-called medullary 

 rays which are quite prominent in cross sections of the trunks 

 of certain kinds of trees. Just within the bark is the cambium 

 ring composed of a layer of cells which are in active division as 

 long as growth is taking place rapidly, especially in the spring. 

 It is here that growth in thickness takes place. This explains 

 why in grafting it is absolutely essential in order to secure a 

 perfect union that the inner line of the bark of the scion should 

 exactly coincide with that of the stock. In budding the little 

 ring of exposed cambium tissues is seated in direct contact 

 with the outer layers of the same tissue of the stock. Hence 

 ideal conditions for a union is thus povided. 



In the leaves the shape of the cells and their arrangem.ent 

 may be compared to the stones of a loosely piled stone wall, but 

 there is a certain amount of order and purpose to it. The upper 

 and lower surfaces of the leaves are covered with a definite pro- 

 tective layer of cells, called the epidermis. Scattered through 

 this are many little mouth-like openings called breathing pores 

 or stomata. These openings are surroimded by two peculiarly 

 shaped guard cells which remind one of lips. These guard cells 

 are so constnicted that they automatically open apart when the 

 leaf is turgid with water and close together when the reverse is 

 the case. The breathing pores not only serve as avenues for 

 the escape of watery vapors from the interior tissues of the leaf 

 but also for the escape of oxygen gas which is a by-product in 



