DISPERSAL OF SPORES 645 
specially prevalent among the earliest fossils. A definite succession of 
sporangia in time and in space is found in the sori of the Gradatae: here 
the physiological drain is not sudden and severe, but it is spread over a 
long time: in Zyichomanes it may be over a period of years. The basal 
position of the youngest sporangia gives them the further advantage of 
being near to the source of supply at the time when they are most in 
need of it, while those which are approaching maturity are successively 
removed from it to a position where they can readily shed their spores. 
In the prevalent type of the Mixtae there is a succession of sporangia in 
time but not in space. The physiological drain is spread as before over 
a long period, so that in this respect the Gradatae and Mixtae appear 
equally practical; but in the mixed sorus the receptacle is found to be 
wide and flat: this has the double advantage of enlarging the surface 
from which nutriment can be derived, and of shortening the distance through 
which it must be transferred. In point of protection those sporangia which 
are more advanced give an adequate protection to those which are younger: 
there are, however, multitudinous minor adaptations to this end which 
cannot be entered upon here. It thus appears that each of these types 
of sorus, the simultaneous, the basipetal, and the mixed, which Palaeon- 
tology shows to have been of successive appearance in the course of descent, 
was a biological amendment upon its predecessor. 
Lastly, the facilities for dispersal of the spores when mature remain 
to be considered. There are three main types of dehiscence: by a slit 
in the median plane, which is characteristic of the Simplices; by a slit in 
an oblique lateral plane, seen in the Gradatae; and by a slit laterally in a 
transverse plane, characteristic of the Mixtae. Dehiscence can only be 
properly carried out when free movement of the mechanical tissue is allowed : 
where, as in the Leptosporangiates, there is a definite annulus and a free 
sporangium, the whole body alters its form on dehiscence: accordingly 
the grouping of the sporangia in the sorus is a determining factor in the 
position of the mechanical ring if it is to be effective. In the case of 
median dehiscence, as in G/eichenia, the sporangium widens laterally, requir- 
ing free elbow-room on either side before the sudden springing of the 
annulus throws the spores out right and left. It thus appears that the 
median dehiscence of an annulate sporangium can only be a practical 
working arrangement where the sporangia are not in close lateral juxta- 
position. oxsoma, with its basipetal sorus, is an exception in having the 
median dehiscence ; but the sporangium is so constructed with its incomplete 
ring that though the dehiscence is median, still the sporangium as a whole 
does not widen on dehiscence; it is, in fact, a compromise, the annulus 
being so modified as to be still compatible with the basipetal sorus. 
With the exception of Matonia, Dipteris, and Plagiogyria, all the Ferns 
showing the oblique dehiscence have basipetal sori. The sporangia overlap 
one another like the shields of the Roman /sfudo, while all are so closely 
packed together that no space is given, as in the former case, for lateral 
