134 THE ROOT 



dissected type of secondary vascular cylinder results (Jeffrey, 

 pp. 156-157 and liayward, pp. 48-51). When secondary growth 

 is pronounced in a root, the primary phloem, cortex and epi- 

 dermis soon become crushed and slough away. In trees, a 

 typical "bark" is produced and, save for the exarch primary 

 xylem in the center, all structural resemblance with a root is lost. 

 The first periderm layer of the root arises by the formation of a 

 phellogen in the pericycle. Later-formed phellogen layers may 

 subsequently appear, as in the stem, from living cells in the sec- 

 ondary phloem. 



II. Material for the Study of the Root. 



1. The root of buttercup (Ranunculus sp.). Obtain a stained 

 trans-section of the root and study the following tissues and 

 regions beginning at the edge of the section : 



(a) The epidermis, a nniseriate but broken layer of collapsed 

 and partially destroyed cells. The imperfect condition of the 

 epidermis presumably is due to the abrasive effect of the soil on 

 the root. Notice that a more or less disorganized protoplast is 

 visible in some of the epidermal cells. 



(h) Within the epidermis occurs the rather broad, homo- 

 geneous cortex which is composed entirely of rather thin-w^alled, 

 "isodiametric" parenchyma cells most of which are separated 

 from one another by prominent intercellular air spaces. Observe 

 that while the outer layers of the cortex are composed of rather 

 tightly joined empty cells (forming a **hypodermis"), the inner 

 cortical cells contain prominent starch grains. Large, somewhat 

 irregidar simple pits are visible on the end walls of the cortical 

 parenchyma cells. 



(r) The center of the root is occupied by tiie stele which is 

 externally separated from the cortex by a uniseriate cylinder 

 of cells, the endodcrmis. Study the endodermis under high 

 power, noting the presence of a protoi)last in many of the cells. 

 The salient feature of tlie endodermis (in llic primary condi- 

 tion) is the presence of a suberized or cutinized band which 

 extends comi)le1cly ;il)()ii1 1h(> inner surface of tlie radial and 

 end walls of each cell. These band-like thickenings of the wall 

 are known as Casparlan strips and in i-ecent years have received 



