24 INTRODUCTION. 



not confined to the trachae, but extends to the whole of 

 the vascular system. 



The whole of the vegetable kingdom consists of mass- 

 es of these several elementary organs, with the exception 

 of fungi, mosses, and lichens, whose vessels are all of a 

 cellular form : they have no vascular system whatever. 



Emily. That affords a strong argument against the 

 passage of the sap through the vascular system. 



Mrs. B. Certainly ; the fibres of plants are compos- 

 ed of collections of these vessels and cells closely con- 

 nected together. The root and stem of plants consist of 

 such fibres : if you attempt to cut them transversely, you 

 meet with considerable resistance, as you must force your 

 way across the tubes, and break them, whilst, if you slit 

 the wood longitudinally, you separate the vessels without 

 breaking them, and have only to force your way through 

 the elongated cellular tissue which connects them. 



Emily. The difference is very observable ; but I won- 

 der that the cells, being formed of a delicate membrane, 

 are not squeezed and crushed to pieces in the stems of 

 plants, especially when they become hard wood. 



Mrs. B. The cells, by the growth of the stem, are 

 frequently drawn out of their original form, and elon- 

 gated ; but the vascular system, which is of the greatest 

 importance, is internal, and lodged in a bed of cellular 

 integument, so that the pressure of the bark or surround- 

 ing parts is not sufficient to crush it. 



The layers of wood which you may have noticed in 

 the stem or branch of a tree cut transversely, consist of 

 different zones or fibres, each the produce of one year's 

 growth, and separated by a coat of elongated cellular 

 tissue, without which you could not well distinguish 

 them. 



The cuticle, which is the external skin or covering of 

 the plant, consists of an expansion of the cellular tissue ; 

 and is furnished with pores for evaporation. 



71. With what exception does the whole vegetable system consist of 

 these elementary organs'? 72. How does Mrs. B. prove, that roots 

 and stems consist of the several vessels named! 73. What difficulty 

 does Emily apprehend to these cells from the growth of the plants! 

 74. How does Mrs B. reply to this suggestion of Emily! 75. Of 

 what is it seen that the layers of Wood consist, when cut transversely! 

 76. What is the external covering of the skin called, and of what does 

 it consist! 



