ON SAP. 75 



This exhalation is not visible, because the water is so 

 minutely divided as to be dissolved by the atmosphere as 

 soon as it comes in contact with it. 



Caroline. It may then be compared to our insensible 

 perspiration. 



Mrs. B. True ; and it is called by many botanists the 

 perspiration of plants, and it sometimes happens (as is 

 the case also with animal perspiration) that it becomes 

 sensible. This occurs only in plants whose leaves have 

 simple ribs uniting at a point at the extremity of the leaf. 

 The sap is accumulated by the absorption of the roots 

 during- the night, and that portion of it which is destined 

 to be evaporated flows towards this sole aperture, and 

 may be seen there in the form of a minute drop, if ob- 

 served before sunrise, for it is reduced to vapor by the 

 first solar rays : the subsequent evaporation being equal 

 to the absorption, no accumulation takes place, and no 

 fluid is perceptible. This effect may be seen on the 

 leaves of corn, which, with all the gramineous family, 

 have simple ribs. 



Caroline. Plants, then, must increase in weight dur- 

 ing the night, since they absorb by the roots without ex- 

 haling by the stomas ? 



Mrs. B. They do so ; and whenever, through any 

 accidental cause, the stomas are obstructed or diseased, 

 the plant becomes dropsical, from the accumulation of 

 the water it has taken in and cannot discharge. Plants 

 growing in vases in a room are very subject to this mala- 

 dy, owing to their not having sufficient light to evaporate 

 freely. 



Emily. Yet if you expose a nosegay in a room to the 

 sun's rays, it withers. 



Mrs. B. Because the sun produces a degree of evap- 

 oration which the poor mutilated flowers are unable to sup- 

 port ; for though the Stalks may be immersed in water, 

 the organs of absorption are wanting, and the quantity of 

 water they suck up is quite inadequate to the evaporation 



406. Why is not this exhalation visible 1 ? 407. What is it called 

 by botanists'? 408. With what plants does perspiration become visi- 

 ble'? 409. When and in what form may it be seen! 410. Why 

 do plants increase in weight in the night"? 411. When do they be- 

 come dropsical! 412. What plants are subject to this malady 1 ? 

 413. Why do flowers separated from their roots wither in the sun's 

 rays 7 



