210 BIOLOGY. 



2. EARTH-ORGAN, VASCULAR TISSUE. 



1062. So long as the vesicles or cells lie as globes 

 upon each other, triangular interspaces are found be- 

 tween them, which stand in conjunction with each other 

 on all sides. As water is found in these interspaces, it is 

 plain that they do not entirely disappear with the trans- 

 formation of the cells into rhomboidal-dodecahedra. 

 These spaces* are called intercellular passages or sap-tubes, 

 Vessels. In many plants, such as those which contain a 

 milky juice, particular sap-tubes run through a part of 

 these intercellular passages, and are probably formed by 

 ^condensation of the sap. Both are therefore in a phy- 

 siological respect of one kind. At bottom also the 

 blood-vessels of animals are naught else but passages in 

 felted cellular tissue. 



1063. As the principal polarity of plants has been 

 directed upwards, and the cells therefore been protracted 

 lengthwise ; so also the chief direction of the vessels is 

 parallel with the axis of the plant. 



1064. The vegetable sap ascends in these tubes, 

 which must be therefore viewed indeed as constituting 

 the earth- or nutritive organ. 



3. AIR-ORGAN, TRACHEAL TISSUE. 



1065. The plant is not merely earth- and water- 

 organism, but also air-organism ; and there must there- 

 fore be developed in it an anatomical system, which coin- 

 cides with the process of air. 



1066. Besides the cells and tubes naught else is 

 found in vegetable tissue but spiral vessels ; what are 

 called scalariform tubes, annular vessels, dotted ducts, 

 vermiform or strangulated vessels, are no peculiar 

 formations in themselves, but only different conditions 

 of the spiral vessels. 



1067. The spiral vessels are the air-system of the 

 plant, and therefore rightly deserve the name of Trachea. 

 They exhibit the structure of the air-tubes in insects, and 

 contain, according to the most authentic observations, 



