PREPARATION OP TISSUES. 255 



bog-moss, and in the elegant creeper Cobcea scandens. In 

 some orchidaceous plants the leaves are almost entirely 

 composed of it. A modification of this form of tissue is 

 found in the testa of some seeds, as in those of Salvia, 

 Collomia grandiflora, &c. 



The curious and interesting sporules of ferns, when 

 ripe, burst, and are dispersed to a distance ; so that they 

 should be gathered before they come to maturity, and 

 mounted as opaque objects. The development of ferns 

 may be observed by placing the seeds in moistened flannel, 

 and keeping them at a warm temperature. At first a 

 single cellule is produced, then a second ; after this the 

 first divides into two, and then others follow ; by which a 

 lateral increase takes place. 



Pollen-grains from most flowers are very interesting 

 objects; the darker kinds show best when mounted in 

 Canada balsam, and viewed as opaque ; the objects more 



Fig. 160. Pollen grains and seeds. 



A, Seed of Clove-pink. B, Poppy seed, c, Pollen of Passion flower (Passiflora 

 ccerulea). D, Pollen of Cobcea scandens. 



transparent in fluid, or even preserved dry, will show 

 better. The prettiest and most delicate forms are found 

 in Amarantacece, Circurbitacece, Malvacece, and Passi- 

 florece; others are furnished from the Convolvulus, Gera- 

 nium, Campanula, Hollyhock, and some other plants. 

 The curious peculiarities of a few are shown in fig. 160. 



