836 THE DISPERSION OF SPECIES BY MEANS OF FRUITS AND SEEDS. 



whilst the outer layers of cells of the seed-coat are not in a state of tension. When 

 the seed is quite ripe the cell-membranes in the strained layer of tissue swell up, the 

 outer layer of the seed-coat, being no longer able to withstand the pressure to which 

 it is subjected, is rent asunder and the edges of the slit thus formed roll suddenly 

 back. A violent jerk is given to the inclosed seed, in consequence of which it flies 

 out through the fissure in the capsule immediately in front of it (fig. 458^). The 

 fact of the ejection of the seeds of Balsamacese also has long been known. The 

 fruit of Imjjatiens Nolitangere, one of the members of that family, is an oblong 

 capsule composed of five carpels (see fig. 459 % The walls of this capsule are con- 

 structed of three layers of cells. The layer lying immediately beneath the epidermis 

 consists of large and highly turgid cells, and is called the turgescent layer. It is in 

 a state of great tension, and when the seeds are ripe and the union between the five 

 carpels gives way along the lines of union, a relaxation of the tension takes place, 

 the loosened tissue of those lines is torn, the five carpels roll up, and their rapid 

 movements of involution result in the expulsion of the seeds contained in the fruit. 

 Cyclanthera explodens and Thladiantha dubia, plants belonging to the Cucur- 

 bitacese, as also several Crucifers of the genera Dentaria and Cardamine, in 

 particular the species shown in fig. 459^ (Cardamine impatiens), exhibit similar 

 phenomena, except that in these cases the carpels do not roll inwards but outwards. 



In the instances hitherto dealt with the cause of the expulsion is the turgidity 

 of cells or the swelling up of cell-membranes with a concomitant maintenance of a 

 state of extreme tension in a particular layer of tissue situated in the wall of the 

 fruit. In the next class of cases the phenomenon depends on the desiccation and 

 consequent contraction of a special layer of the fruit-wall which leads to a rupture 

 and subsequently to a bending over and rolling up of particular parts of the fruit. 

 This change is accomplished with great rapidity and has the effect of hurling away 

 the seeds or the separate parts of the fruit or even the entire fruit itself. We will 

 only mention some of the best known instances of this kind. 



The fruit of the Marsh Crane's-bill (Geranium palustre; see fig. 459^) has a 

 5-angled column rising up in the centre of a circle of five carpels. The carpels are 

 hemispherically inflated at the base, and terminate above in long bristles or beaks. 

 Each contains a single seed. When the seeds are ripe, the tissue composing the 

 beaks undergoes desiccation, which, however, is not of uniform intensity throughout. 

 The outer layer, consisting of several plates of succulent cells, dries up more quickly 

 than the inner layer, which is composed of thick- walled cells. The result is that the 

 beak lifts itself away from the axial column, and curls up externally like a watch- 

 spring. No resistance to this movement is afforded by the delicate dried tissue 

 which has hitherto served to hold the carpels together, and as the cavity of each 

 carpel is open along the inner surface, and the seed lies in it simply as though it 

 were resting in the hollow of a hand, the rapid drawing up of the beak has the effect 

 of ejecting it in a wide curve away from the carpel (see fig. 459^). In the Marsh 

 Crane's-bill, as also in the other large-flowered species of the genus Geranium, the 

 tops of the beaks continue attached to the axis, and the latter, together with the five 



