INFLUENCE OF EXTERNAL STIMULI. MECHANICAL STIMULI 269 



Analogous cases occur amongst the lower plants. Riccia fluitans exhibits a land- 

 form and a water-form. The former has numerous hair-roots, the latter has usually 

 none ; but if the water-form be allowed to swim upon a fine sieve it also produces the 

 hair-roots, and it is evident their usual suppression upon it is due to the want of 

 a contact-stimulus in the water 1 . 



We can say the same of the Fungi. The mycelial threads 2 of Peziza tuberosa 

 and P. sclerotiorum 3 form tufts of hyphae in consequence of contact, and these 

 serve as anchoring organs ; similarly Mucor stolonifer 4 produces rhizoids on 

 its stolons. Biisgen 5 found in different parasitic fungi that they only produced 

 anchoring organs when they touched some firm body ; the formation of haustoria, on 

 the other hand, was usually the result of a chemical stimulus 6 . 



Borge 7 has investigated the formation of anchoring organs in some Chloro- 

 phyceae amongst the Algae. The species of the genus Spirogyra exhibit very 

 different behaviour. Some have the capacity to form rhizoids under special 

 external conditions, others do not possess this power. To the former belongs 

 Spirogyra fluviatilis, which can grow fixed upon stones in flowing water. The 

 rhizoids which act as its anchoring organs arise in consequence of contact, but 

 other conditions may induce their formation, for example, cultivation in a solution of 

 cane-sugar or of urea of a certain concentration 8 . It is noteworthy that all the cells 

 of the filament, which has not a polar construction, possess the capacity to form 

 rhizoids, but the cells forming them must be terminal cells, or at least lie in the 

 neighbourhood of a terminal cell. Should dead cells occur in the middle of a fila- 

 ment, the adjacent living cells are to be considered as terminal cells. In Vaucheria 

 clavata the capacity to form rhizoids is limited to the germ-plants, and is 

 stimulated to activity in these by contact. If these rhizoids be cut off, the wound 

 heals, and then long threads grow out which, notwithstanding contact, do not 

 form rhizoids, but in other filamentous Algae, for example, species of Cladophora, 

 Draparnaldia glomerata, rhizoids may arise without contact, and if these be cut off 

 new ones can be developed to take their place. The adhesive disks of Plocamium 

 (Fig. 45) doubtless arise in consequence of contact, and they behave quite like those 

 of Ampelopsis. 



It is difficult to determine whether we have to do with mechanical or with chemical 

 stimuli in the fertilization-processes which we observe in the flowers of dicotyledonous 

 and monocotyledonous plants ; it is most probable they are chemical. At the 

 time of pollination in such plants as Corylus, Alnus Quercus, and their allies, there is 

 no sign of the placenta in the ovary, far less of the ovules, and in the Orchideae the 

 ovules in most species are indeed laid down at the time of pollination, but are still 



1 Regarding Marchantia, see Pfeffer in Arb. d. bot. Instituts in Wiirzburg, i. p. 77. 



2 Brefeld, Untersuchungen a. d. Gesammtgebiete d. Mykologie, iv. p. 112. 



3 De Bary, in Botan. Zeitung, 1886, p. 382. 



* Wortmann, in Botan. Zeitung, 1881, p. 385. 



5 Biisgen, Uber einige Eigenschaften der Keimlinge parasitischer Pilze, in Botan. Zeitung, 1893, p. 53. 

 The literature is cited here. 



6 See also Miyoshi, in Botan. Zeitung, 1894, p. i ; Id. Pringsh. Jahrb. xxviii. 



7 Borge, Uber die Rhizoidenbildung bei einigen fadenformigen Chlorophyceen, Upsala, 1894. 



8 0.5-0-25 per cent, in the case of cane-sugar, 0-4-0-2 per cent, in the case of urea. 



