LECTURE XL. 



roots turn away from the light and attach themseh-es closely to the vertical 

 substratum. 



In Agarics which grow up from the soil of woods, the stalk, which is funda- 

 mentally nothing other than a shoot-axis, places itself vertically upright, though the 

 umbrella-Hke pileus is horizontal, and the lamellar, tubular, or conical hymenial pro- 

 jections on which the spores are produced, and situated on the under side, are directed 

 downwards. I showed in i860 that this depends on geotropic sensitiveness on the 

 part of these Fungi ; for if an Agaric which is still growing is placed in a horizontal 

 position, the stalk erects itself vertically, till the pileus is horizontal ; whereas if this 

 erection is prevented, or if it does not follow quickly enough, the lamellae, tubes 

 or conical outgrowths of the hymenium make energetic curvatures downwards like 

 the ends of primary roots. 



Even stemless Agarics, such as those frequently growing out from the trunks of 

 trees, exhibit a similar anisotropy in their various parts. As shown in Fig. 393, Agarics 

 grow out from a prostrate horizontal tree trunk from the upper side as well as from 



the flanks, but always so that the ste- 

 rile substance of the pileus itself strives 

 to assume a horizontal position, while 

 the spore-forming hymenial outgrowths 

 are directed vertically downwards. 



Exactly similar facts of anisotropy 

 are met with in non-cellular plants, e. g. 

 the Mucorini, which are so instructive in 

 other respects also. If the spores of a 

 Mucor or Phycoviyces are sown on a cube 

 of damp bread B, which, as in Fig. 394, 

 is fixed by means of a needle N' N to 

 the lid of a large glass cylinder, the 

 bottom of which is covered with water, 

 the root-like mycelium extends itself in 

 the substance of the bread, and after 

 a few days slender sporangiophores arise on all the surfaces of the cube ; 

 those on the horizontal upper surface of the cube of bread at once grow verti- 

 cally upwards, those on the horizontal lower surface and on the vertical sides, 

 at first stand out at right angles to these surfaces, as will appear later, but sub- 

 sequently they erect themselves geotropically as shown in the figure at /\ The 

 mycelium of the Fungus, however, behaves exactly like a root-system : after it has 

 permeated the nutritive substratum, individual branches of the mycelium also come 

 out from the lateral surfaces of the cube of bread, but more particularly from its 

 horizontal lower surface, and grow thence downwards into the damp air, while their 

 lateral branches assume oblique directions. 



Now, whether the parts of plants grow upwards, downwards, horizontally or 

 obliquely, depends by no means upon their so-called morphological nature. Organs 

 which, and with good reason, receive the same names in descriptive botany, may 

 nevertheless assume entirely different directions of growth in different species of 

 plants. Shoot-axes, as has already been stated, may grow vertically upwards, 



Fig. 393. — Transverse section of the trunk of a tree, o 

 Fungi (of the genus Thelephora) are growing, P—P' 



