100 DIVISION I. GENERAL MORPHOLOGY. 



When ripe, and usually some time before they are ripe, they have a firm cell- 

 membrane which may in very many cases be divided into two layers, an outer layer, 

 the episporium or exosporium, and an inner, the endosporium, each of which may be 

 itself stratified. In delicate or small spores the separation into two- layers is very 

 difficult to recognise, or cannot be recognised, before germination, and in some cases 

 it never becomes apparent (Exoascus) ; in the latter case the cell-wall, often wrongly 

 termed episporium, is a simple colourless or a coloured membrane. 



In the numerous cases in which the two layers are to be distinctly seen, the 

 outer one is usually a firm membrane, of various colours and various shades of colour, 

 rarely quite colourless, and usually gives its colour to the whole spore. The surface 

 is either quite smooth, as in most of the teleutospores of Puccinia and in many 

 Pezizeae, or is more commonly furnished with thickenings projecting outwardly in the 

 shape of warts, spikes, wrinkles, or a net-work of ridges, which vary in thickness and 

 height in the different species, from the very delicate punctiform elevations of the 

 gonidia of Puccinia coronata, Eurotium, &c. and reticulations of Peziza aurantia and 

 Puccinia reticulata to the extremely thick warts of Genea, the spikes of Tuber 

 melanosporum, Octaviania and Triphragmium echinatum, and the anastomosing ridges 

 of Tuber aestivum. The outer layer is in these cases either homogeneous or stratified. 

 In acrogenously formed spores a thin outermost lamella is often distinguishable from 

 the rest, and is shown by the history of development to be the original delicate 

 membrane of the spore-primordium, which developed with the spore, while the other 

 lamellae are formed on its inner surface. The prominences on the surface of the spore 

 often belong, as in uredospores and Corticium amorphum, exclusively to this outer 

 envelope, which may be called the primary lamella. The compound or septate spores 

 also are generally enclosed in the wall of the mother-cell, which developes with them 

 as a close-fitting sac (Fig. 51 /). The episporium of some spores shows a differ- 

 entiation perpendicular to the surface into portions of unequal density, a striation 

 or areolation, along with or instead of the stratification. Fischer von Waldheim 1 has 

 observed this in the Ustilagineae. The episporium of the aecidia of Phelonites 

 strobilina, Peridermium Pini, Caeoma pinitorquum, Chrysomyxa and other Uredi- 

 neae is particularly beautiful, appearing as if composed of small prismatic rods of 

 denser substance perpendicular to the surface of the spore and connected together by 

 narrower bands of a less dense and more transparent substance (Fig. 50). The 

 convex extremities of the rods project outwards like warts. The structure is best 

 seen when the episporium is made to swell by the application of sulphuric acid 2 . 



The endosporium is usually colourless or at least much paler than the coloured 

 episporium, and is smooth and homogeneous or stratified ; it is generally distinguished 

 from the episporium by greater softness and delicacy, but by no means always by 

 less thickness. 



Some spores have a number of pores or pits disposed in regular order on their 

 surface ; the number of these is usually definite, but may vary within narrow limits in 

 the same species. Many of these pores serve as places of exit for the tubular outgrowths 

 from the spore at the time of germination, and may therefore be termed germ-pores ; 



1 See sections LVI and LVII, and the literature cited there. 



2 See Reess, Die Rostpilzformen d. Coniferen ; also Hot. Ztg. 1879, p. 803. 



