208 
Beer. — Stitdies in Spore Development. 
as 1892 (and again in 1907) (9), described chromatic shells of this kind 
enveloping the nucleolus in several animal tissues, and his observation has 
been confirmed by other zoologists. 
Cavara’s (3) observations upon nucleoli may also be recalled in this 
connexion. This author described the nucleoli of higher plants as consist- 
ing of two parts : an external chromatic layer and an inner mass of plastin. 
During this period of protoplasmic growth an intine has been formed 
on the inner surface of the pollen-wall. This layer is very thin over the 
general surface of the pollen-grain, but at each exit pore it is greatly 
thickened and protrudes towards the exterior. Where it is thickened the 
intine can clearly be seen to be composed of a number of lamellae, which 
suggests that its growth has taken place by the apposition of successive 
layers of material. An extension of the delicate outer layer of the exine 
covers the external surface of each protrusion of the intine at the exit pores 
of the pollen-grain (Fig. 17). The intine stains, although not very intensely, 
with those dyes which are characteristic of pectic bodies. Treated with 
calcium-chloride-iodine solution it gives at first no reaction, but after 
remaining in the solution for some days it is found to have coloured faintly 
violet. A preliminary boiling with dilute acid and alkali, according to 
Mangin’s method, yielded no clearer cellulose reaction with the iodine 
reagents. From these reactions it may be concluded that the intine consists 
of pectic bodies associated with a little cellulose. The exine has meanwhile 
grown in thickness, and the relation of its parts to one another can now be 
very clearly seen. 
The thickening bands of the exine have increased greatly in breadth 
and thickness, so that they now form a massive layer only perforated by the 
relatively small exit pores. This layer, which may be called the * mesospore ’ 
according to Fitting’s (7) terminology, possesses the reactions of a cuticular- 
ized structure (Fig. 17). 
The outermost lamella of the exine (which we already saw at an 
early stage as an extremely delicate membrane) still remains very thin, and 
it can now be seen to possess an open structure perforated by countless 
little apertures which give it the appearance of a very fine reticulum in 
surface views. This perforated structure of the lamella is well seen 
in Fig. 1 8 and, in section, in Figs. 17 and 9. At the exit pores this reticulate 
layer dips down and covers over the protrusion of the intine. As at an 
earlier stage, we still find that the rodlets are limited to a hexagonal system 
of bands corresponding to the originally hexagonal disposition of the 
thickening bands (Fig. 18). At the angles of each hexagon is usually 
a spine. Both the spines and the f heads ’ of the rodlets pass through the 
perforated outer lamella to reach the outer surface. The spines are 
spindle-shaped structures ; their internal portions or roots are composed of 
two prongs, as already seen at an early stage. Fig. 18 shows that these 
