66 
MILDEWS, RUSTS AND SMUTS 
tubes pass through the minute holes into the prepared 
food, where they continue to grow, or they do not pass 
through the holes, but perish of hunger on the surface of 
the mica. An extended series of experiments proved that 
different fungi were attracted, or would grow into one par¬ 
ticular kind of food only, and refused to enter into, and grow 
in any other kind of food. This is more especially true 
of parasitic fungi, saprophytes, being less specialised, are 
not so discriminating as to the nature of their food. This 
power on the part of parasitic fungi to discriminate in their 
choice of food, is called chemotaxis, or chemtropism, and a 
food that attracts the germ-tubes of a fungus is said to be 
positively chemotropic, whereas a substance that does not 
attract the germ-tubes is said to be negatively chemo- 
tactic to that fungus. From the above experiments it 
is assumed that when a spore alights on a given leaf, and 
germinates, if the cell-sap of the plant contains a substance 
suitable for the growth of the germ-tube ; that is, if the 
sap of the leaf is positively chemotactic, the germ-tube 
enters the leaf and forms a mycelium, whereas if the sap 
of the plant is negatively chemotactic, no infection takes 
place. 
UREDINACEAE 
All the species are obligate parasites, that is, they can 
only • obtain their food from living plants. A vigorous 
septate mycelium is present, which wanders in the tissues 
of the host-plant. The spore-forms connected with re¬ 
production, are as follows, given in the sequence of their 
appearance. Pycnidia (=spermogonia), producing very 
minute spore-like bodies, considered to be of no functional 
value at the present day. Aecidia, producing spores in 
chains, and enclosed in a peridium ; these spores give 
origin to the next stage in the cycle of development, uredo- 
spoxes, which are often produced throughout the period of 
vigorous growth of the host-plant. Uredospores are always 
i-celled, and are capable of germination the moment they 
are mature. Germination consists in the protrusion of 
one or more germ-tubes, which enter a host-plant, and grow 
directly into ordinary mycelium. Teleutospores, the last 
spore-form in the cycle of development, may be i-celled 
or many-celled, and are characterised by their remarkable 
product of germination, which consists of a promycelium 
bearing promycelium spores. As a rule teleutospores 
germinate during the spring following their production, 
