268 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



the spores are sown in a solution of less nutrient value, especially if they 

 are submerged in a fermentable sugar-solution, and are restricted in their 

 supply of air. There appears then a copious formation of cross-walls, 

 and the single cells which thus arise increase by sprouting. Similar 

 processes are found in some other forms, but we cannot further discuss 

 the subject l . 



D. MECHANICAL STIMULI 2 . 



Mohl was the first to show that the adhesive disks on the tendrils of certain 

 species of Ampelopsis appear in consequence of contact with a firm body (Fig. 130). 

 We have here to do with a contact-stimulus. Different species of Ampelopsis 

 behave differently. Some, like A. hederacea, possess ordinary tendrils which twine 

 round a support and eventually become firm woody structures, but if they do not 

 happen to find a support they die off at an early period. Ampelopsis quinquefolia, on 

 the other hand, fixes itself to walls and tree-trunks, by means of adhesive disks on its 

 tendrils 3 , but these can also act like ordinary tendrils. In tendrils which do not 

 come into contact with a firm body, no viscid disks appear ; the primordia of these 

 are nevertheless so far apparent in that the epidermal cells of the convex side of the tip 

 of the tendril are elongated in a radial direction a condition which does not occur 

 in the tendrils of other species of Ampelopsis. By the strong elongation and the 

 periclinal division of these after the contact-stimulus has been applied, as well as by 

 an elongation in radial direction of the cells lying immediately beneath the epidermis, 

 the formation of the adhesive disk is brought about. The adhesion is caused by 

 the disk adjusting itself to all the inequalities of the surface of the substratum and 

 secreting a sticky substance 4 . It is interesting to note that in Ampelopis Veitchii the 

 formation of the adhesive disks has proceeded a considerable stage before a contact- 

 stimulus can operate ; the primordia of the disks are visible as small swellings on 

 the tips of the tendrils, but their further development into adhesive organs is 

 dependent upon a contact-stimulus. 



The tendrils of some Bignoniaceae, for instance, Bignonia littoralis, B. capreo- 

 lata, Hanburya mexicana, and according to Naudin 5 those also of the cucurbitaceous 

 Peponopsis adhaerens, form in like manner adhesive disks in consequence of contact- 

 stimuli. It is worthy of note that the trifid tendrils of the bignoniaceous genus Haplo- 

 lophium end in smooth shining adhesive disks before contact with the support ; 

 when they adhere they sometimes become very broad 6 . Von Mohl first showed that 

 the haustoria of Cuscuta arise in consequence of a contact-stimulus which can 

 induce their formation on all sides of the shoot-axis 7 . 



1 See Zopf, Die Pilze, p. 271, in Schenk's Handbuch der Botanik, iv. 



2 A comprehensive discussion of this subject falls more within the sphere of Physiology than of 

 Morphology ; but I cannot altogether pass it over here. 



3 See the figure in Sachs, Lectures on the Physiology of Plants, p. 667. 



* See A. von Lengerken, Die Bildung der Haftballen an den Kanken einiger Arten der Gattung 

 Ampelopsis, in Botan. Zeitung, 1885. 



5 Naudin, in Ann. d. Sciences Nat. ser. 4, xii (1859), p. 89. 



6 Fritz Muller, Notes on some climbing plants near Desterro, in Journ. Linn. Soc., ix. p. 348. 



7 See also Peirce, A contribution to the physiology of the genus Cuscuta, in Annals of Botany, viii. 



