162 



DISEASES OF TREES 



comes almost black. The lower leaves of each shoot are dwarfed, 

 while the upper ones develop normally. If one infects a healthy 

 cowberry plant with the aecidiospores of the columnar rust of the 

 silver fir which will be described presently the stem remains 

 unaltered during the first year, although the mycelium spreads 

 in the tissues of the cortex. Next year, however, the young 

 shoots are affected in the manner just described. The my- 

 celium grows into the young shoots, where, by the exuda- 

 tion of a ferment, it 

 stimulates growth in 

 all the cortical cells. 

 This effect, however, 

 can only be produced 

 so long as the cells 

 of the new shoots are 

 still young. But on 

 account of the slow 

 upward growth of 

 the mycelium in the 

 shoot, it only reaches 

 the apex at a time 

 when the cells of 

 the cortex are com- 

 pletely matured, and 

 when, consequently, 

 it is no longer able 

 to stimulate to in- 

 creased growth. 

 The mycelium, 



FIG. 92. Cortical parenchyma and epidermal cells 

 from the stem of V. Vitis Id&a. The mycelium 

 grows in the intercellular spaces, and pushes short 

 branches, which swell at the apex, against the outer 

 wall of the cells. A delicate prolongation at the 

 apex of these branches penetrates the cell-wall, after 

 which a sac-like haustorium develops in the interior 

 of the cell. Underneath the epidermal cells the 

 hyphoe enlarge in a clavate manner, a a. Haus- 

 toria, b, and teleutospore-mother-cells, c c, develop 

 in the epidermal cells. Magnified 420 times. 



however, pushes up 



as far as the topmost bud, which may be stimulated to shoot 

 out even in the same year as that in which it is formed. The 

 mycelium, which is perennial, is intercellular, and abstracts 

 nutriment from the parenchymatous cells by means of haustoria 

 (Fig. 92). It ultimately reaches the epidermis, underneath which 

 it swells up in a clavate manner (Fig. 92, a a). 



Haustoria (#) are also pushed into the epidermal cells, which 

 may at once be distinguished, by their shape, from the young 

 spore-mother-cells (c c), which are also developed there. 



