268 ON THE NATURAL SYSTEM 



of ascertaining the degree of consequence of each. The 

 first is its utility : this is the most general, and would be 

 sufficient of itself, were we so perfectly acquainted with 

 vegetable physiology, as to judge of the importance of an 

 organ, by its degree of utility in the economy of a plant. 

 Thus we may safely conclude, that an organ essential to 

 the life of a plant, is of a higher order than one of which 

 it can be deprived, without sustaining any material injury. 



Emily. But are the functions of different organs not 

 sufficiently known, to enable us to judge whether they 

 are more or less essential to a plant? 



Mrs. B. Not always ; but in a doubtful case, there are 

 two other rules by which you may be guided. Generally 

 speaking, the greater the number of plants in which the 

 same organ can be found, the greater is the degree of im- 

 portance that ought to be attached to them. For instance, 

 the calyx may be pronounced to be an organ of much 

 greater consequence than the involucrum, because a much 

 greater number of plants have a calyx than an involucrum. 



Emily. I understand that perfectly. 



Mrs. B. There is besides a third criterion. There are 

 certain organs which exist, or which are wanting, in all 

 the plants of the same family ; and others that are only oc- 

 casionally found in plants, otherwise very similar to each 

 other. The first of these organs are naturally of much 

 greater importance than the latter, as they appear to be 

 indispensable to the system of organisation of these plants. 

 Were you to ask me, for example, whether a stipula or a 

 thorn were of greatest importance, I should not hesitate 

 to say the stipula, for the reason I have just assigned. 



Emily. When you judge of the importance of an organ 

 by its degree of utility in the economy of a plant, how can 

 you compare organs adapted to functions of a completely 

 different nature ? In the animal frame, it would be diffi- 

 cult to determine whether the lungs were a more useful 

 organ than the stomach, the eye than the hand does 

 not the same difficulty occur in the vegetable structure ? 



1444. How many ways are there of ascertaining the importance of the 

 organs of plants'? 1445. What is the first, and how is it explained'? 

 1446. How does the number of plants in which any particular organ is 

 found effect its utility! 1447. How is this illustrated by the calyx and 

 volucrum 1 ? 1448. What is the third criterion 1 ? 1449. How is 

 that illustrated by the stipula and thorn 1 1450. What question does 

 Emily ask, indicating the diffic^ 7 y of ascertaining the utility of organs by 

 comparison'? 



