802 



THE DISTRIBtrriON OF SPECIES BY OFFSHOOTS. 



considerable an increase and distribution as Strawberries. The accompanying 

 table gives the length of runners and shoots of some well-known species in which 

 the formation and rapid distribution of offshoots is particularly noticeable on 

 suitable substrata. 



In those cases in which plants change their position by the development of 

 oifshoota in any direction, whilst they die off in the opposite one, progress is always 

 restricted. The offshoots penetrate only by slow degrees in the surrounding soil, 

 and many years elapse before a space of 100 metres is traversed in this way. The 

 change of position is much more rapid when the offshoots become detached from 

 their place of origin and are carried to a new spot by special mechanisms of transit, 

 by currents of water, the wind, or finally by the help of men or animals. In this 

 way it may happen that single detached cells, cell-groups, buds, and shoots may be 

 carried vastly further than 100 metres in a few minutes, through long valleys, over 

 steep precipices, or even over high mountain ridges. This rapid distribution is not 

 indeed so certain in its result as the slower mode of progression. It may easily 

 happen that the wind or water current lands the detached offshoot on some spot 

 where further development is impossible, where it must inevitably perish. Appa- 

 rently, however, this disadvantage is compensated for by the immense quantity of 

 such detached offshoots. Again, there are plants which form two kinds of offshoots, 

 those which propagate slowly but surely, which are few in number, and others, 

 developed in large numbers, which are distributed rapidly but less certainly. 



Only a very small proportion of plants develop offshoots which after they 

 become detached reach a new locality spontaneously, by means of special organs of 

 motility. This class of brood-body is always aquatic and of very small size, and 

 its development can only be followed under the microscope. The best-known 

 examples are Fungi, belonging to the Saprolegniaceoe and Chytridiaceje, the 

 dark green Vaucherias, and other species of Algae. The Saprolegnias are sapro- 

 phytes growing in and on the bodies of animals which have died in the water — 

 not only fish, crustaceans, and insect larvae, but also birds. They form delicate, 

 thread-like, tubular hyphae, which ramify repeatedly, and part of which penetrate 

 into the corpse like a root-plexus, while the rest rise up above the body in the form 

 of white or grey felt, which floats in the water. Single tubular erect hyphae 

 assume a knob or club-shaped form, and their protoplasm divides up into numerous 

 portions. Ultimately the club-shaped tube opens at the apex, and the little proto- 



