VASCULAR TISSUE. 15 



to which Schleiden gave the name of cytoblast (kiitos, a cell, 

 and blastos, a germ), a nitrogenized body. As soon as the cyto- 

 blast has attained its full size, there appears upon it a fine 

 transparent vesicle. This is a young cell, which continues to 

 swell out and increase in size till the cytoblast is only a minute 

 body, imbedded in the side of the wall, or sometimes loose in 

 the cavity. The cytoblast is sometimes absorbed after the 

 growth of the cell, and at others it is permanent. 



21. There is a mucilaginous layer on the inner surface of the 

 cell, which, with the cytoblast, seems to control all its vital 

 functions. It is a nitrogen compound, and has been called the 

 internal utricle. 



Section 2. — Vascular Tissue. 



22. Vascular tissue consists of tubes whose length generally ex- 

 ceeds several times their breadth. There are various kinds of it. 



Dotted Ducts, Fig. 13, are formed of a series of short 

 cylindrical cells, placed end to end. In their young 

 state they may be separated into the individual cells 

 which compose the tubes ; but as they advance in 

 age, the separating membrane closing the ends of the 

 cylinders is ruptured, thus forming a continuous tube. 

 This may be distinctly seen, under favorable circum- 

 stances, in the young vine, hickory, or oak, where 

 the membrane may be seen ruptured in some cases on ratted ducts. 

 one side, leaving the membrane attached to the 

 other side, assuming somewhat the appearance of a valve ; 

 in others it is ruptured in the center — the membrane co- 

 hering to the sides of the tube. They are the largest of the 

 vessels, and are scarcely found in any other situation than in the 

 wood. They are very distinct in the beech, oak, vine, and 

 hickory, being the largest pores observed on a transverse section 

 of these several kinds of wood ; but in the pine, and trees of 

 the same family, they are never found. This form of tissue 

 derives its name from rows of dots regularly arranged on its 

 surface. Sometimes the individual cells are very distinct, and 

 bear some resemblance to a string of beads, when it is called 

 the moniliform tissue. 



7 A 



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What is the cytoblast ? How is the cell formed from it ? What becomes 

 of the cytoblast ?— 21. What layer on the interior of the cell? What 

 has it been called? — 22. What is vascular tissue ? How are dotted ducts 

 formed? How correspond with other vessels in size? Where found? 

 In what trees not. found ? From what does it derive its name? What ia 

 tho moniliform tissue ? 



