104 HEART-ROT 



desired direction. It is expressed botanically by saying 

 that the pores are positively geotropic, i. e. they grow 

 towards the centre of the earth. The necessity for exactitude 

 in this orientation is demonstrated by the following calcula- 

 tion. If a tube is O25 mm. across and bears spores for 

 a distance of 2 cm. from the orifice, a displacement from 

 the vertical of tan'^fo (= s ), i.e. a displacement of 

 less than 1, will prevent any spores growing as far as 2 cm. 

 from the orifice from escaping (fig. 42). Actually the 

 accuracy has to be far greater than is shown in this calcula- 

 tion, since the spores do not simply fall from the basidia 

 but are shot off to a distance of rather less than 0-1 mm. 

 (vide Buller). The ejection mechanism for the Polyporeae 

 has been investigated by Buller, and, as is shown in the 

 accompanying diagram (fig. 41, B), the ejection is accom- 

 plished by the splitting of the septum between the spore 

 and the sterigma. The force of propulsion is provided by 

 the turgidity of the basidium and the spore, each of which 

 tends to bulge out the septum which separates them. The 

 fineness of the mechanism is due to the thinness of the 

 sterigma and the consequent small cross -section of the 

 septum that crosses it. 



The spores, being very small, present a large surface in 

 proportion to their weight, so that once they have escaped 

 from the pores they are easily carried away by the wind. 

 They germinate at once, either in pure water or damp air, 

 or in nutrient solution, and under suitable conditions every 

 spore germinates. The germ tube may emerge from any 

 part of the wall, even, occasionally, from the small point 

 where the spore was attached to the sterigma. 



2. Conidiophores arise in all cultures. They are borne 

 on especially broad hyphae, which occur in groups in the 

 mycelium, and have the form shown in fig. 43. They may 

 be simple or branched, and have a few cross septa, and the 

 whole conidiophore is somewhat reminiscent of that of 

 Aspergillus. The conidiophore is rather swollen at its apex, 

 and the conidia closely resemble the basidiospores. When 

 the conidia have been ejected the conidiophores are left 



