56 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
in tlie secondary cortex, and to the second half of this group. The 
course of the tertiary increase in thickness is, however, somewhat more 
simple than that described by Yan Tieghem. 
(4) Structure of Organs. 
Structure of an Assimilating Parasite.* — Herr H. Hackenberg 
describes the vegetative structure of a phanerogamic parasite, Cassytha 
americana, belonging to the Lanraceae. The slender stems are leafless, 
like those of Cuscuta, and attach themselves to the host by means of a 
somewhat similar structure, but contain abundance of stomates, and are 
distinguished by the remarkable peculiarity of the assimilating tissue of 
the cortex being very fully developed. In this respect it resembles the 
chlorophy lions parasites belonging to the Santalaceae and Ehinanthaceae, 
which are, however, mostly root-parasites. But its mode of life is that 
of Cuscuta. For when a haustorium has once been formed, all direct 
connection with the soil ceases ; it lives on the sap of the host, which 
it gradually kills ; the lower part of the parasite itself perishes, while 
the upper portion continues to develope. 
Membrane of Pollen-grains.f — M. L. Mangin gives the following 
as the result of his investigations on the membrane of ripe pollen- 
grains : — (1) The membrane is difi’erentiated into two layers; the one 
external and cutinized, the extine; the other internal and always 
present, the intine. (2) The intine, of which the structure is sometimes 
complex, is always formed of a combination of cellulose and pectic 
compounds ; but the cellulose is limited to the internal face of the 
intine, and the j^ectic compounds form, nearly in a state of purity, the 
mass opposite the pores and hitherto considered as cellulose. In 
Spartium junceum the extine will be found to be composed of two 
layers : an internal cutinized zone, which is coloured yellow by alkalies ; 
and this zone is clothed by a very thin colourless membrane, which is 
difficult to see. The intine also shows clearly two distinct layers. 
(3) When the membrane of the pollen-grain swells, it is the pectic 
compound which becomes soluble and absorbs water, and form a gela- 
tinous mass, and later a viscid liquid. The cellulose does not take any 
part in this. (4) A callus which up to the present time was only 
known to exist in sieve-tubes has been found in a certain number of 
pollen-cells (Coniferae, Cyperacese, and Juncaceae), as an intercalary 
mass between the extine and the intine, and more or less mixed with 
substances composing this latter membrane. In Carex riparia the 
callus will be seen to be non-homogeneous ; but it shows stratification, 
which is caused by cellulose and pectic substances forming bands 
parallel to the internal face of the mass. 
Thickening-layers of Pollen-grains.f — Herr A. Tomaschek has in- 
vestigated the phenomena attending the growth of pollen-tubes, in the 
case of pollen-grains of Colcliicum autumnale made to germinate on the 
cells of ripe fruits. He states that when the grains are made to 
germinate in nutrient solutions, the growth is so rapid that abnormal 
Verhandl. Natur. Ver. Preuss. Eheinl., xlvi. (1889) pp. 98-138 (2 figs.), 
t Pull. Soc. Bot. France, xxxvi. (1889) pp. 274-83. 
t Bot. Ctntralbl., xxxix. (1889) ))p. 1-6 (11 figs.). 
