HYGROSCOPISM. 2S7 



The hygroscopic character of some of the species 

 of Selaginclla is famihar to horticulturists. These 

 plants have somewhat the appearance of large 

 mosses, and are not uncommon in greenhouses. In 

 the classification there is one entire section devoted 

 to the species which have the foliage curved inwards 

 when dry, so that many of them roll up by contrac- 

 tion into the form of a ball. This is the habit of 

 Selaginella convahita, a species abundant in Bahia 

 and Pernambuco, and also of Selaginclla lepidopJiylla, 

 which latter has been called the " Resurrection plant," 

 from its habit of expanding under moisture. One of 

 the earliest observers of this phenomenon in South 

 America was the celebrated traveller Martius, who 

 called the plant Lycopodiuni hygroinetriann. 



Sensibility to variations in humidity is also an 

 important factor in the dispersion of the spores in 

 many of the ferns. In these the sporangia are girt 

 by an elastic ring, which assists in the rupture of the 

 sporangium. " When the sporangia arrive at ma- 

 turity, and are under certain favourable conditions as 

 to dryness, the elasticity of the ring causes them to 

 burst open with force and sound sufficient to be heard, 

 and this takes place in a direction at, or very near to, 

 a right angle with the direction of the ring." ^ This 

 serves to remind us that the bursting of the spathe 



' Smith, " Ferns, British and Foreign," p. 51. 



