176 
Parasitic Aphelenchi 
5th day. Lower leaves from four plants, (a), (b) and (c) were negative, (d): in the tissues 
of one lower leaf four larvae found. 0-25, 0-3, 0-32, 0-465 mm. long; soil infected 45 days 
previously; the leaf axils were unfortunately not examined, but it is probable that they 
contained adults which were the source of these larvae, a chrysanthemum was however 
also growing in the pot. 10 th day. In a low growing leaf many adults and larvae found; the 
pot had been infected 41 days previously, and had been bare of vegetation for three 
days before the planting of this specimen, the adults must therefore have lived at least 
three days, and almost certainly 31 days in the soil before attacking this plant. 
8th, 13 th and 26th days. Adult Aphelenchi in the leaf axils or lower leaves. 28 th day. Adults, 
larvae and eggs found in the leaf axils of the lower 20 mm. of the stem, adults and larvae 
also in the tissues of the lower leaves. 
Marcinowski (1910) with begonias planted in infected soil, found that leaves in contact 
with the earth were invaded. 
We can therefore conclude that adults invading a plant from the soil may 
take up ectoparasitic life in the leaf axils up to a height of at least 2 cm. 
above ground, and may here deposit eggs; the larvae hatched from them 
then proceed to the more extended invasion, and to their definitive habitat 
in the mesophyll. On the other hand, leaves close to the ground level may be 
invaded direct from the soil, the worms becoming entoparasites forthwith. 
This brings us to the passage from ecto- to entoparasitic life, and the mode 
of entry into the spaces of the mesophyll: 
D. Aphelenchi having reached the leaf surface enter the 
MESOPHYLL SPACES THROUGH THE STOMATA. 
This statement rests on the following observations: (1) Aphelenchi from orchid leaves, 
dropped in water on the ventral surface of leaves of an inverted Begonia, entered the leaves; 
application of infected to healthy leaves gave the same result (Marcinowski, 1908). (2) 
Aphelenchi have been observed traversing the stomata (Osterwalder, 1902 [quoted by 
Marcinowski], Marcinowski, 1908). 
Molz (1909) held that the worms did not enter through the stomata, but through wounds 
of the plant surface, on the ground that (1) the stomata were too small to admit the para¬ 
sites, and (2) leaf to leaf infection succeeded only with infected leaves. On the other hand, 
(1) my measurements of Chrysanthemum stomata give larger apertures (viz. 0-02 x 0-01 mm., 
and the shorter diameter can be increased by the forcible abduction of the guard cells), and 
the size of the parasite varies with the size of the stomata ( vide bottom of p. 162); and (2) the 
negative result of Molz’s leaf to leaf infection is explained by conditions of atmospheric 
humidity (Marcinowski, 1910, vide infra (E)). 
» 
E. Influence of atmospheric humidity, and of previous disease 
OR INJURY OF THE HOST PLANT, IN ASSISTING MIGRATION 
OF THE PARASITE. IMMUNITY. 
Marcinowski (1908, 1910) records three important experiments on the influence of damp 
on the migration of Aphelenchi. (1) A portion of infected chrysanthemum leaf was mounted, 
ventral surface uppermost, in water on a slide; many worms were observed to creep out 
through the stomata. (2) If infected plants are placed in a humid atmosphere the parasites 
wander out, and adopt ectoparasitic life in the leaf axils. (3) Leaf to leaf infection experi¬ 
ments in begonias succeed in a humid atmosphere under a bell jar, but fail in the open air. 
When chrysanthemum leaves are moistened by heavy dew, adult Aphelenchi can be ob¬ 
served (under a binocular microscope) wandering freely over their hairy surface (Molz, 1909). 
