IN VEGETABLE BIOLOGY. 379 
genus, the long and slender stalk being scarcely enough to warrant 
generic rank. However, it may possibly be a type allied to Apio- 
cystis, which, although descended from Volvocineous ancestors, 
has lost its external cilia, and so betrays to-day no sign of its 
origin, its relation to meroccenobial and holoceenobial forms being 
to some extent comparable to that borne by some fixed Tunicata 
to Appendicularia and craniate Vertebrata respectively ; but this 
cannot be decided until Physocytium has been made the subject 
of further investigation. 
DESCRIPTION OF THE PLATES. 
(Unless otherwise stated, the magnification is 400 diameters.) 
Prare LIV. 
Fig. 1. Earliest fixed condition: a, with large, b, with smaller cilia. 
2. 'The gonidium has divided by a transverse septum. 
3. Four-celled, and fig. 4, eight-celled state. 
5. Small zoosporangium with somewhat larger cells than ordinary. 
6. Subspheroidal zoosporangium, the stalk wanting. 
7. Curious form consisting of four pockets upon a common stalk. 
8. Zoospores swarming inside the zoosporangium ; one of them in the act 
of passing out through a hole in the side. 
9. The zoosporangial wall breaking down before the escape of the zoo- 
spores: a, proximal part of wall still retaining its cellulose cha- 
racter; 5, the proximal part broken down, and many of its zoospores 
already escaped ; y, a zoospore swimming away. 
10. A zoospore come to rest upon a Cladophora fracta thread ; it has lost 
its cilia, but has not yet secreted a wall. 
Prate LV. 
Fig. 11. Form with grouped cells: ant, a group of “ antipodal gonidia " left at 
the bottom of the stalk. 
12, a-e. Details in the division of the gonidia, X 600. 
13. Large zoosporangium with grouped gonidia (these drawn only at the 
bottom): a, space left after passage of a group of gonidia—as a 
ccenobium ? 
14. Lobed zoosporangium, only a few of whose gonidia are drawn; on 
the right-hand side is a row of gonidia surrounded by a very deli- 
cate investing wall. 
15. Zoosporangium with coenobia standing out from its wall. 
16. Small zoosporangium which has already emitted at least one 
zoospore, 
