EXPLANATION OF THE PLATES. 363 



of wood-cells a ; and porous-cells b, the so-called vessels of the 

 wood. 



FIG. 9. A fine transverse section of the same wood. In this 

 also, the small but very thick- walled wood- cells a are readily distin- 

 guished from the large but proportionately thin vascular cells b. 

 At c are perceived some other rows of peculiar cells, called by 

 vegetable anatomists medullary rays, by joiners " silver grain," 

 which run through the wood, radiating from the pith to the 

 bark. 



FIG. 10. A delicate longitudinal section of the wood of the 

 common Pine, consisting of very long porous wood -cells, which are 

 distinguished by a peculiarity of the pores. They exhibit two 

 circles, a large outer one a, and a small inner one b ; a peculiarity 

 which only occurs in this particular form in the wood of Cone- 

 bearing trees, and which enables us to distinguish this even in the 

 condition of coal or fossil wood. 



PLATE III. 



MOST OF THE OBJECTS IN THIS PLATE ARE MUCH ENLARGED ; 

 BUT WHEN THIS IS NOT THE CASE, IT IS EXPRESSLY 

 DENOTED BY THE LETTERS " N. S." (NATURAL SIZE). 



FIG. 1 . Development of a reproductive cell of a Conferva, which 

 frequently occurs as a green filamentous slime in stagnant water. 

 a. The spore (reproductive cell), b. First stage of development; 

 the spore has sent out a thin, tubular process, c. Second stage ; the 

 process has become elongated, and a new cell has been formed at the 

 opposite end of the spore, d. Third stage ; the young plant has 

 become attached by its elongated process to a fragment of wood, and 

 is gradually growing out at the other extremity into a perfect filament, 

 by continually forming fresh cells. 



FIG. 2. Development of the spore of a Fern. a. The spore, 

 which in this case does not consist of a simple reproductive cell, but 

 has a peculiar dark- coloured coat. b. First stage ; the cell has 

 broken through the coat and become elongated in a tubular form. 

 c. Second stage ; in the protruded end of the tube several cells have 

 been formed and are already green, but the original cell remains 

 within the dark- coloured coat. d. Third stage ; the green cells have 

 become so much multiplied that they form a little roundish leaflet, 



