now PLxVNTS GKOW. 169 



still a special force, and not to be resolved into any other sort or combination of 

 attractions or repulsions, whether culled electricity, osmose, or any other 

 name." 



In an excellent Manual of Botany recently published by Eobert Brown in 

 England, he says, " the cause of the ascent of the sap is by no means placed 

 beyond a doubt." The eminent German, Hugo von Mohl, one of the best 

 of authorities who has investigated these subjects, places the circulation of sap 

 among the unsolved questions. 



It is, and always has been thought to be, more difficult to explain the descent 

 than the rise of sap, and yet our students are asked by our county superin- 

 tendents of schools to explain the circulation of sap in plants, and Wood in 

 his text-book, extensively used in our schools, is supposed to explain it fully 

 in about half a dozen pages. 



RESPIRATION. 



As was shown by the table, oxygen is given off in the light by foliage and 

 all green parts of plants. Carbonic acid (CO') is decomposed, the carbon re- 

 tained and the oxygen given back to the air. By this process plants are taking 

 up what is given off by animals and what is deleterious to them ; they give off 

 oxygen, which animals must have for breathing. This is only one of the 

 points in which plants and animals mutually aid each other. This is the 

 beautiful " compensation " notion referred to last evening by Mr. George Parme- 

 lee. It is not yet exploded, although as before stated and shown on the chart, 

 certain parts give off carbonic acid, and the leaves allow some to run to waste, 

 or go back to the air at night. 



There is no fact better established in the growth of plants than this: that 

 plants take in carbonic acid by their leaves, decompose it in the light, retain- 

 ing the carbon and giving off oxygen. Less carbonic acid is given off iy 

 healthy growing plants than is absorbed. This is beyond controversy. 



Fungi, that is toad-stools, puff balls, moulds, and the like, are plants which 

 do nothing, — or assimilate nothing, — but only use that which other plants have 

 ■ collected from the mineral world. They are scavengers, — parasitic in one 

 sense. 



THE EVAPORATION 



of moisture from the leaves is well known. It passes off in large quantities 

 from growing plants, at all times, in air not saturated with moisture. 



In the greenhouse at the Agricultural College, thrifty plants of the colocacia, 

 or caladimn, a common, large-leaved plant, have often been seen to give off 

 water in little drops, coming from a small hole at the end of the leaf. The 

 water comes into two little veins, one along each side of the leaf, and finally 

 meets at the top, where the drops may be seen to escape with a little force. 

 Other plants have been found to do the same thing. The water has never been 

 analyzed to my knowledge. This is not new. Several pitcher plants secrete a 

 liquid in their pitchers and drown insects, of which they make a rich liquid 

 manure. The plants are especially contrived for catching insects by stiff hairs 

 pointing down. Some of them secrete a sweet substance about the mouth of 

 the pitcher to entice the flies and bugs iuto the fatal pit, where they are drowned 

 and eaten up by the plant. 



Herbert Spencer thinks he has discovered some apparatus in the leaves to 

 serve as glands to collect the assimilated sap and then return it to the parts 

 below. How the sap is assimilated in the leaves no one pretends to know. 

 22 



