4 SCIENCE AND FRUIT GROWING 



way through the cortex into the soil. The structure and develop- 

 ment of these is similar to that of the main root, and, as time 

 goes on, the plant becomes furnished with a widely ramifying 

 system of roots, provided with multitudes of very fine rootlets, 

 each of which is constantly forming new root-hairs, which more 

 than counterbalance the loss of those from the older parts of the 

 roots. When the tip of a root is broken off, that root cannot grow 

 any more in length, as it has lost its growing point, and in such 

 a case, fresh laterals are produced from the root, or even from 

 the lower parts of the stem. These stem-roots are said to be 

 adventitious, and an excellent example is seen whenever a cutting 

 is " struck/' whereby a new plant is formed from the portion of 

 a stem inserted in the soil. 



STEM. The main function of the stem of a plant is to act as 

 a support for the branches, and as a channel for the translocation 

 of foodstuffs between the roots and the leaves. Its structure is 

 specially adapted to this end, for the size and strength of the 

 stem are adjusted to the burden of leaves that it has to bear, 

 and, therefore, as will be seen later, to the amount of work it 

 is called upon to carry out in connection with the process of 

 nutrition. 



A transverse section of a young stem of a dicotyledonous 

 plant (Fig. 2) such as an apple shows 



(1) An outer layer of regularly shaped cells the epidermis 

 of which the outer wall is usually thickened or corky, forming 

 the cuticle. Scattered over the epidermis are numberless pores, 

 or stomata, which communicate with the interior of the stem, 

 and play an important part in the functions of breathing and 

 feeding. 



(2) A broad band of thin- walled cells forming the cortex, 

 bounded on the inner side by the endodermis with thickened 

 side walls. The outer layers of cortical cells contain granules 

 of green colouring matter, called chlorophyll. 



(3) A stele, or central cylinder, the outer layer of which is the 

 pericycle. Within the pericycle is a ring of vascular bundles, 

 each consisting of a group of xylem and phloem, connected by a 

 layer of actively dividing cells called the cambium. The phloem 

 is on the outside, and the spaces between the bundles are 

 occupied by thin-walled ground tissue, which also fills up the 

 middle of the cylinder, and is called the pith. 



As the stem gets older various important structural changes 

 occur. The cambium ultimately extends across the ground 

 tissue between the bundles, and, as it continues to divide, throwing 



