670 A TEXTBOOK OF THEORETICAL BOTANY 



Anatomy of the Root 



The root of the Pine resembles that of a Dicotyledon, but it is on the 

 whole simpler. The apical meristem produces no leaves, and it also differs 

 from that of the stem in cutting oft" a series of cells forwards as well as back- 

 wards. These cells are short-lived, but they form the protective root cap, 

 which acts as a buffer between the delicate cells of the meristem and the 

 hard particles of the soil. The young root is clothed with a zone of short 

 root hairs, which are thin-walled prolongations of the cells of the external 

 or piliferous layer. The root hairs in the Conifers are not well developed 

 and soon disappear. Within the piliferous layer is a wide zone of parenchy- 

 matous cortex which is bounded on the inside by the endodermis. This 

 layer is composed of suberized cells, usually impregnated with tannin which 

 gives them a brownish orange colour. 



The stele is limited by the pericycle, which may consist of six or more 

 layers of cells, which are also frequently impregnated with tannin and 

 contain starch grains. There are generally two protoxylem groups, which lie 

 at the ends of an ellipse of metaxylem in the centre of the root. The 

 development of this metaxylem proceeds from the two protoxylem groups 

 towards the centre, so that in a very young root some of the central cells 

 may still be found unlignified. The phloem forms two arcs, on each side of 

 the metaxylem, and separated from it by parenchymatous cells. This 

 separation of the xylem and phloem is characteristic of root structure in 

 general. Each protoxylem group is slightly forked, formed a Y, and 

 between the branches lies a large resin canal. 



Often, even before the primary tissues have been completely organized, 

 secondary thickening begins by the development of a cambium from the 

 parenchymatous cells between the phloem and the metaxylem (Fig. 669). 

 This cambium cuts off secondary xylem towards the centre and secondary 

 phloem towards the outside, so that two masses of secondary tissue are 

 formed, separated by two wide medullary rays, one opposite each protoxylem. 

 Later the cambium extends itself across these rays, outside the resin canal, and 

 the extension of the secondary wood gradually encroaches on the primary rays 

 until they are reduced, in the course of one or two annual rings, to the width 

 of a single cell, when they become indistinguishable from the secondary rays. 



As soon as the cambium has become active a layer of the pericycle com- 

 mences to function as a cork cambium, and cuts off a layer of cork cells 

 towards the exterior. As this process continues a thick layer of cork is formed 

 separating the cortex from the stele, and thus being deprived of food by this 

 barrier of cork, the cortex soon dies and disappears. From the beginning of 

 cork formation the older part of the root ceases to be an organ of absorption 

 and functions only as a conductor of watery material absorbed by the young, 

 actively growing end of the root. 



Although generally there are only two protoxylem groups in the root of 

 the Pine, when it is said to be diarch, occasionally three are found, when the 

 root is spoken of as triarch, and higher numbers, up to six occur. 



