THE ANGIOSPERMAE : ROOTS 791 



consists of adventitious roots arising from the stem, which entirely replace 

 the main root system of the Dicotyledon type. Monocotyledons are thus on 

 the level of the Ferns rather than of the Gymnosperms in this respect. 



The first group of these adventitious roots to appear are all coequal in 

 growth and are all positively geotropic, but their branches follow the same 

 rules of growth as in Dicotyledons, producing a fibrous system. Branching 

 is never on the same prolific scale as in Dicotyledons, and in a few Mono- 

 cotyledons, for example in Arum macidatum and in many Orchids, it is 

 entirely absent. The root systems in Monocotyledons are therefore on a 

 relatively small scale, even when the plant, as in the Palms, is tree-like. 



Many Monocotyledons have underground stems. Where these grow 

 horizontally they produce adventitious roots throughout their length, but 

 contracted stems, such as bulbs, produce successive crops of roots from the 

 base, each crop lasting only a single season. 



Summary of Elementary Structure.* 



If one examines the transverse section of a young root (Figs. 786 and 788) 

 an inch or so from the apex, one may note the following typical arrangement 

 of the tissues : — 



1. A surface layer of cells, the piliferous layer, most of whose cells bear 



on the outer side tubular prolongations, the short-lived root hairs, 

 which may be several millimetres long and have very thin walls. 



2. A layer of cells next to the piliferous layer, which becomes the surface 



layer when the former has been shed. This is called the exodermis. 



3. A broad cortex of parenchyma cells, usually filled with starch grains. 



4. A well-marked endodermis. 



5. A rather small stele, enclosed by the endodermis. 



6. The stele is surrounded by a layer of thin-walled cells, the pericycle, 



which lies next to the endodermis. 



7. A number of groups of xylem cells, with the protoxylem outwards, 



/.e., exarch. 



8. An equal number of groups of phloem cells, alternating with the 



xylems and having their protophloems outwards. 



9. A small central area of parenchyma, the conjunctive tissue, which 



is sometimes replaced by metaxylem. 



Dicotyledons usually have two (diarch), four (tetrarch), or five (pentarch) 

 xylem groups, less frequently three (triarch) or seven (heptarch), and rarely 

 more. Monocotyledons have greater numbers (polyarch), sometimes as 

 many as thirty, surrounding a relatively large conjunctive tissue (Figs. 787 

 and 789). 



This statement of elementar}' structure will serve as a basis for the 

 following more detailed consideration of anatomy. 



* This section and the corresponding sections on the stem, the leaf and the flower are 

 intended as outlines for the benefit of the elementarj' student. 



