74 FAMILIAR LESSOXS IN BOTAXY. 



189. Some botanists style them air vessels, because they 

 contain air ; but as investigation has proven them to con- 

 tain fluids also, this term cannot be considered strictly 

 correct. Their offices have been as much a matter of dis- 

 cussion as their name. Some affirm that they are breathing- 

 tubes; others, that they convey fluids to recently developed 

 tissues. Darby believes that both these theories are true, 

 that in their earliest stage they contain fluids, and in more 

 advanced age, air. Chemical analysis has proved this air 

 to contain seven or eight times more oxygen than the air 

 we breathe. 



190. If you cut a stem of water-lily under water, air will 

 be seen rushing up in great bubbles to the surface. The 

 most important functions that these vessels thus far are 

 known to serve is the pi'otection of the plant in the earliest 

 stage of development. Xo other form of vessel could so pre- 

 serve the plant from the injurious effects of wind and rain 

 as this spiral tube, which above all other forms combines 

 the great requisites of strength and elasticity. There is 

 another tissue which consists of minute tubes connected 

 with each other, forming a network of branching veins. 

 It extends throughout the jDlant, but is more abundantly 

 developed in the liber of the bark. It is called lactiferous 

 tissue, from the Latin lac, milk, because it contains the muci- 

 lagenous substance called latex, the milky juice of the plant. 



191. Botanists disagree as to the character of this fluid, 

 some consider it highly elaborated sap; others, that so far 

 from being sap fit for the use of the plant, it is a substance 

 eliminated totally unfit for use. It is this fluid that exudes 

 from fig stems when you pluck the unripe fruit, and is seen 

 in abundance in the euphorbia on our prairies. 



189. What do some botanists call these vessels ? Why? What are the theo- 

 ries respecting them ? What does Darby say? 



190. If you cut a stem of the water-lily, what will you see ? What has an 

 analysis of this air shown ? What is the most important function of these tubes ? 

 Is there another tissue? Describe it. 



191. Do botanists disagree concerning this fluid also? 



