6 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



Ducts are closed 50 when the spires touch each other ; annular, when they seern 

 to consist of separate rings 17 ; reticulated, when the spires cross each other 19 ; 

 scalariform, when the lines upon their sides are horizontal and equidistant ; 

 septate, when the interior is divided by pierced disks, as in Echinocactus. 



34. Ducts occur among the woody tissue of herbaceous 

 plants ; are abundant in the wood of the higher tribes of cel- 

 lular plants, such as Ferns and Lycopodiacese ; and their ends 

 are often in immediate connection with the loose cellular tissue 

 occupying the extremities of the roots. 



35. Their functions have not been accurately determined. 

 It is probable that they act as. spiral vessels when young ; but 

 it is certain that they become filled with fluid as soon as 

 their spires are separated. 



36. LATICIFEROUS TissuE 18 (Cinenchyma) consists of uninter- 

 rupted anastomozing tubes, whose final divisions are so deli- 

 cate, that the eye only discovers them when aided by the 

 most powerful microscopes. It forms the proper vessels of 

 old writers. 



37. It principally occurs in the liber of Exogens (124), 

 whence the ramifications proceed to the surface of all the 

 organs, and penetrate the hairs, where they form a most 

 delicate network. 



38. Laticiferous tissue conveys latex, a peculiar fluid, usually 

 turbid, and coloured red, white, or yellow ; often however 

 colourless. 



39. The use of this tissue is to carry the latex to all the 

 newly formed organs, which are supposed to be nourished by it. 



The large trunks of Cinenchyma are vasa expansa, or opopkora ; the small are 

 rasa cm: tract a. 



40. There are no other elementary forms of tissue. Air- 

 vessels, Reservoirs of oil, Lenticular glands, are all either dis- 

 tended intercellular passages, or cavities built up with cellular 

 tissue, or large cells filled with peculiar secretions. 



41. When such cavities are essential to the existence of a 

 species, they are formed by a regular -arrangement of cellular 

 tissue in a definite and unvarying figure ; Ex. Water-plants. 

 When they are not essential to the existence of a species, 

 they are mere irregular distensions or lacerations of the tissue : 

 Ex. Pith of the Walnut-tree. 



42 All these forms of tissue are enclosed within a skin 

 lied the epidermis, which is one or more external layers of 



