506 SUMMARY OF CURRENT RESEARCHES RELATING TO 
the portion of the mycele in contact with the protoplasm of the host- 
plant. Sporangioles, vesicles, and hyphal clumps are organs of nutri- 
tive interchange. The changes in the host-plant are of two kinds, 
degeneration of normal structure, and the formation of special cells for 
the accommodation of the Fungus. The velamen of terrestrial and 
aerial roots (Orchidese, &c.), and the trichomes of coralloid structures, 
are devices which facilitate absorption by endotropic Fungi. Myco- 
rhizal Fungi sometimes penetrate non-absorbing organs. The acquisi- 
tion of the habit of symbiotic saprophytism renders a group of plants 
extremely unstable as to specific characters. The penetration of non- 
absorbent organs by the fungal symbiont affords opportunity for the 
utilisation of these organs for absorption in case of a diminished supply 
of food from the ordinary sources. There are but few constant ana- 
tomical characters indicative of partial or complete saprophytism. 
Absence of chlorophyll is the only invariable accompaniment of holo- 
saprophytism ; while degeneration or alteration of the stele is not 
always consequent. In the formation of mycorhiza, the fungus may be 
associated with prothallia, roots, or stems. 
Variation in the Graft, and Inheritance of Acquired Characters.* 
M. L. Daniel gives the result of a very large number of experiments 
on the grafting of herbaceous and woody plants. The phenomena of 
grafting afford no exception to the general rule of the influence exerted 
on the development of a plant by its external conditions, and especially 
by those connected with the supply of nutriment ; but this influence is 
not invariably exerted. These acquired characters of the graft may be 
exhibited only in the seed and embryo, and the author sums up strongly 
in favour of their perpetuation by heredity. The effect of grafting 
may be either the fixing and perpetuation of varieties or races already 
in existence, or — and this is especially the case with herbaceous plants 
— it may be the direct production of new varieties, and hence the 
actual improvement of the race. 
Physiology of Roots.! — Herr A. Rimpach classifies roots under four 
heads, viz. (1) Nutrient Roots, whose sole function is the conveyance of 
food-material to the rest of the plant. The central bundle, which con- 
sists chiefly or exclusively of conducting elements, is surrounded by a 
comparatively insignificant cortical parenchyme, which may entirely 
disappear. (2) Firm Attachment-Roots. These do not store up food- 
material, are not contractile, and the conveyance of food-material is so 
unimportant that their sole or chief function may be regarded as the 
fixing of the plant to the substratum. They are characterised by the 
large development of stereome, and are especially characteristic of the 
epiphytic Bromeliaceae, Aracese, and Cyclanthese, and of the terrestrial 
Bromeliaceae, Gramineae, and Palmae. (3) Contractile Roots J ( Zug - 
wurzeln). These contain few or no stereids, while the thin-walled 
parenchyme is relatively well developed and permanent. Contractile 
roots may or may not be also organs of storage. (4) Storage-Roots. 
The structure of these roots agrees with that of other organs used for 
* Ann. Sci. Nat. (Bot.), viii. (1898) pp. 1-226 (10 pis. and 19 figs.). 
f Ber. Deutsch. Bot. Gesell., xvii. (1899) pp. 18-35 (1 pi.). 
X Cf. this Journal, 1897, p. 551. 
