io8 PLANT PHYSIOLOGY 



or lateral growing point, and hence an unchangeable periodicity would be 

 established for internal reasons. Were it possible to compel a bulbous plant 

 to go on forming leaves continuously, or a potato to form only orthotropic 

 shoots with foliage-leaves and to cease the production of stolons with tubers, 

 we should have altered the essential characters of the plant and produced anew 

 species altogether. 



Reference to the alternating origin of foliage and scale leaves suggests the 

 qualitative changes in the productive capacity of the plant, while those we 

 have already discussed, though closely related to this series, have rather been 

 quantitative in character. 



349, 1. 33, for tracts read bracts 



350, 1. 5, for pass to read accumulate in 

 1. 9, for 234 read 187. 



I. 25, after 1893 read; URSPRUNG, 1904). 



351, 1. 38, after leaf-formation read (WiESNER, 1906). 



353, 1. 43, for in the form of read enclosed in 



II. 44-5, for and the cell-mass ... or bud read and this cell-complex may, 

 to a certain extent, take the form of a growing point or of a bud. 



1. 47 P. 355, 1. 45, for Ulothrix zonata . . . uniform system, read Vaucheria 

 repens is an alga forming green layers on moist earth. Examined under the 

 microscope it is seen to consist of long cylindrical, occasionally branched 

 threads, in which usually no transverse walls occur ; the plant is unicellular. 

 Reproduction is effected by two kinds of organs, by swarmspores and by ova. 

 In the formation of swarmspores the end of a branch is cut off by a transverse 

 wall, and the contents of the abstricted portion contract, escape through an 

 aperture in the membrane, and move about by aid of minute cilia arising 

 over the surface of the, as yet, naked protoplasmic mass. Formation and 

 movement of these swarmspores takes place only in presence of water. After 

 a certain time the spore comes to rest, fastens itself firmly to the substratum 

 and germinates into a new filament, which, later on, branches, and which can 

 live both in water and in moist air. In the other mode of reproduction two 

 kinds of organs are formed, known as oogonia and antheridia. Both organs 

 are separated from the thallus by transverse walls, and both open later by 

 apertures at their apices. The antheridia only permit their contents to escape 

 after these have divided into numerous small, colourless spermatozoa, which 

 hasten to the oogonium and penetrate through the opening. One of these 

 sperms then fuses with the protoplasm of the oogonium, which has meanwhile 

 contracted into an egg-cell. Thereafter the ovum excretes a new, thick wall, 

 and becomes an oosperm, which, after a resting period, germinates into a new 

 Vaucheria filament. 



A quite similar kind of reproduction occurs in many Fungi belonging to 

 the genus Saprolegnia. Here also we have a unicellular branched vegetative 

 body, though it contains no chlorophyll. Saprolegnia occurs usually on dead 

 aquatic insects, and the thallus permeates in the first instance the body of the 

 insect. After a certain time it grows out on the surface, and forms all round 

 it a series of radiating threads. The ends of these filaments are, as a rule, 

 segmented off, and form, not one swarmspore, as in Vaucheria, but many, which 

 escape and move about. Later there appear oogonia and antheridia, whose 

 structure we need not describe. We need only note that such an oogonium 

 contains several ova, each of which becomes an oosperm, in certain types at 

 least, only after fusion with the contents of an antheridium. The fact that in 

 many cases the ovum can germinate directly is of less importance in this 

 relation. 



