57 



CHAPTER V. 



THE VASCULAR TISSUES. 



In describing tte elements composing the conducting tissues of 

 plants it must be remembered that similar elements may occur 

 in all of the four great groups, Dicotyledons, Conifers, Mono- 

 cotyledons, and Pteridophyta ; but it is convenient, when 

 considering the vascular tissues, to take those groups in which 

 the greatest variety of conducting elements occur, and in this 

 respect, the Dicotyledons and Conifers afford much the widest 

 scope for investigation. Moreover, by doing this a more rational 

 sequence will be preserved, seeing that it has just been shown 

 in the preceding Chapter how the xylem and phloem arise in 

 Dicotyledons and Coniferse from the cambial layer ; and, in 

 addition, certain other important tissues occur in Dicotyledons and 

 Conifers, such as the medullary rays, endodermis, and pericycle, 

 which, although not strictly speaking vascular tissues, are neverthe- 

 less included in the central cylinder, and have important functions.* 

 In some instances the other groups — viz., Monocotj'ledons and 

 Ferns — possess conducting elements which are important to study, 

 and these will be incidentally described ; but, in the majority of 

 cases, it will be found that Dicotyledons and Coniferse possess in 

 their vascular system a sufficient variety of conducting-element 

 to enable the student to gain a very fair idea of the more 

 important of these. 



Therefore, in the following description of the component 

 elements of the vascular tissues, the order of examination set 

 forth below will be found convenient : — 



(A) The Phloem [produced by the cambium (Dicotyledons and 

 Coniferae) upon its outer aspect]. 



(B) The Xylem [produced by the cambium (Dicotyledons and 



Coniferse) upon its inner aspect]. 



* The endodermis and pericycle occur also in Monocotyledons and 

 Pteridophyta, both being present in the roots of either group ; and an 

 endodermis is to be found round each of the separate bundles in the 

 rhizomes of Pteridophyta. 



