MacDongal : uistrhju iio.\ oK iiazoumofskva roijusta. 171 



for the expulsion of the seeds from tlie berries without the in- 

 tervention or cooperation of outside factors. The expulsory 

 mechanism is best seen in a longitudinal section of the berry. 

 The base of the berry is joined to the stalk by a scission layer 

 several cells in thickness. The outer coat is firm and smooth, 

 and is composed of an epidermal layer with the outer wall ex- 

 tremely heavy and cuticularized. Beneath the epidermis is a 

 mass of parenchymatous tissues, the outer portion of which is 

 slightly palisaded and containing chlorophyll, the inner layer 

 showing only starclies and sugars. Immediately internal is the 

 fibrovascular framework which fuses at the apex in a solid mass 

 of mechanical tissue. Lying inside the fibrovascular strands, 

 and continuous with the parenchymatous tissue external to it is 

 also a mass of similar thin-walled elements of ovoid or cylindri- 

 cal form rich in carbohydrates. These cells have their axes at 

 right angles to the surface of the berry. The second layer in- 

 ternal to the fibrovascular tissue is the expulsory layer, consist- 

 ing of very long thin-walled cylindrical tubes with their axes 

 parallel to the long axis of the berry at the apex of the seed or 

 variously inclined from this position according to the location, 

 but all so arranged that their longitudinal expansion would tend 

 to force the seed out of the mouth of the sac formed by the 

 berry. Immediately coating the seed is a layer of globoid cells 

 with thick mucilaginous contents. The seed has the form of a 

 modern rifle bullet, conical at the basal end and truncate at the 

 apical end, with a general cylindrical outline. The scission 

 layer appears to cut into the mucilaginous layer or at least very 

 nearly so in the mature berry. During the ripening period the 

 contents of the expulsory layer undergo such chemical changes 

 as to give the contents a very high isotonic coefiicient. The 

 consequent osmotic attraction of water into this layer sets up a 

 turgescence which could not be measured, but which probably 

 amounted to many atmospheres. The steady increase of the 

 turgidity of the expulsory layer brings the tension to the breaking 

 strain of the scission layer, and its sudden and complete rupture 

 permits the full force of the pressure to act upon the seed, send- 

 ing it to a distance of two or three meters. The entire arrange- 

 ment is that of a mortar cannon. 



The muzzle of the gun is sealed by the stalk, and the charge 

 amounts to several atmospheres, which is allowed to act upon 

 the seed when the muzzle is freed. The firing of this unique 



