200 BOAED OF AGRICULTURE. 



Explanation of Plates. 



Plate I. represents ihe vai'iations of pressure as indicated by the mercu- 

 rial gauges on the 21st of April, 1873, observations having been taken 

 every hour from 1 A. M. to 12 p.m. Eveiy vertical line marks an hour, and 

 every horizontal line an inch on the column of mercury. Zero represents 

 the point where there is neither j^ressure outward nor suction inward. 



The line A shows the record of the sugar-maple, which at midnight 

 exhibited a suction equal to -6 feet, and at 7 a. m. had increased this to 

 -22.9 feet. As soon as the sun warmed the tree, the mercury began to rise, 

 and at 9.15 A. M. had reached 16.3 feet. Then it declined very gradually, 

 till at 12 P. M. it was at -3 feet. The temi^erature at 7 A, M. was 37° F., at 

 2 P. M. 50.1° F., and at 9 p. M., it was 39.-5° F. 



The line C marks the fluctuations of the mei'cury in the lower gauge 

 of the black birch, which was at the level of the ground, and the line B 

 shows the pressure in the upper gauge, which was placed 30.2 feet above 

 the lower one. The remarkable fall indicated as occurring at 12.45 p. m. 

 was caused by boring into the tree near the ground for the purpose of 

 determining whether the tree was acting simply as a cylinder of water 

 filled by a force from beneath, as seemed evident from the correspond- 

 ence between the two gauges. The reduction and restoration of pres- 

 sure from simply opening and closing the orifice were so rapid and ex- 

 traordinary as to lead to the conclusion that the force operating to produce 

 the pressure was simply the absorbent i)ower of the roots, and this led to 

 the application of a gauge directly to a root Avith the marvellous result 

 already described. 



Plate II., Fig. 1, is designed to illustrate the rotation of the contents of 

 large cells, such as are observed in some aquatic plants. 



Fig. 2 represents the tip of a growing hair on the stamen of a flower, 

 showing the sej^arate cells, Avith protoplasm and nucleus in each, and the 

 beginning of division in the contents of the upper one for the develop- 

 ment of a new cell. 



Fig. 3 is a zoospore or motile germ of a unicellular alga, furnished 

 with two cilia, by the sj)ontaneous movements of which it swims through 

 the water. 



Fig. 4 is the mature plant containing protoplasm, nuclei, and chloro- 

 phyl, and enveloped in a membrane of cellulose and a coating of muci- 

 lage. 



Fig. 6 represents a section both vertical and horizontal of a branch 

 of a sugar-maple, two years old, as it appears in December. The 

 portion included in the lines marked A is of the first year's growth ; 

 those marked B indicate the wood of the second year; while those 

 marked C inclose the three layers of the bark. D represents the pith of 

 loose cellular tissue ; E the pith rays or silver grain of hard cellular tissue 

 connecting the pith with the green or middle layer of bark, which also 

 consists wholly of cellular tissue ; F marks the outer or corky layer of the 

 bark, which is composed of diy, dead cells, which are formed of consecu- 

 tive layers from the outer portion of the living green layer ; G is the 

 green layer of cellular tissue. H shows the liber or inner bark, made up 



