CHAPTER VII. — PHENOMENA OF VEGETATION. — PARASITES. $6$ 



In some endophytes, Phytophthora omnivora ', Tuburcinia Trientalis 2 , Proto- 

 myces macrosporus 3 , the entrance of the germ-tube of the parasite into the cells of 

 the host is more narrowly localised within the limits assigned above. While most 

 perforating endophytes make their way into the interior of the cells of the host at any 

 spot on their outer surface, the three Fungi above named make their entrance at the 

 outer edge of the side wall which divides two epidermal cells, and then grow on in 

 the middle lamella of this wall, splitting it in two and so pressing transversely or obliquely 

 through the epidermis; ultimately they produce both an intracellular and an 

 intercellular mycelium. This is at least the prevailing mode of penetration in these 

 species; perforation of the outer wall and lumen of an epidermal cell occurs 

 exceptionally in Phytophthora. 



In some of the purely epiphytic Fungi which do not penetrate into the host, 

 some for example of the Chytridieae 4 and the Laboulbenieae, the spores when 

 conveyed by some method of dissemination adhere simply to the surface of the host, 

 which is large in comparison to the parasite. The Lichen-fungi which live on small 

 and usually unicellular Algae put out germ-tubes which embrace the cells of the host, 

 as will be described in section CXV, when they encounter them in their elongation. 

 It has never been observed that the direction of this growth is influenced by the host 

 before contact. 



Kihlman 5 has recently observed a very remarkable arrangement for fastening on 

 the host in the case of Melanospora parasitica, which is epiphytic on species of Isaria. 

 The almost cylindrical brown-walled spore, which is 5-6 n in length, germinates by 

 the emission of a germ-tube at each extremity, the tubes, whether grown in water, or 

 in nutrient solutions, being scarcely longer than the transverse diameter of the spore. 

 If the spore lies against or on a hypha of Isaria, which is most frequently the case in 

 a state of nature, the germ-tube becomes firmly attached to the hypha of the host and 

 then developes into a mycelium. If the germ-tube comes into contact with an older 

 hypha of Melanospora, the membrane which separates them is dissolved and they 

 coalesce with one another. But if a germinating spore lies at some distance from a 

 growing hypha of Isaria, and it is not difficult to procure this in plants grown 

 on a microscopic slide, the direction of its growth in length is deflected towards 

 the spore till it comes in contact with the germ-tube, which then unites with it 

 and begins to develope. The greatest distance at which the germinating spore can 

 influence the direction of growth of the hypha is from four to five times the length of 

 the spore. 



The physiological analysis and explanation of all these phenomena of 

 aggression, adhesion, and penetration through openings and membranes has yet to be 

 undertaken. We can here only notice briefly some of the chief points to be 

 considered. 



The facts which have been stated above with regard to the perforation show, on 

 the one hand, distinct effects produced by the germ of the parasite on the host. The 



1 R. Hartig, Arbeiten d. forstbot. Instit. Miinchen, I. 



2 Woronin, as cited on page 185. 



:1 Wolff in Bot. Ztg. 1874. 

 1 See above, p. 171. 

 5 As cited on page 262. 



