Ci AS EAR SHR 25 
this means the stream of water and salts will be tapped and 
the deficiency in root-hairs will be rectified. Let us consider 
the difficulties in the way of such penetration. Roots are 
protected externally by a bark largely composed of cork. 
Added to this, there are frequently thick-walled cells in the 
cortex, which vary from isolated bundles to uninterrupted 
bands of considerable thickness. The’ rest of the cortex 
consists of parenchyma, frequently starch-laden, and there 
may or may not be a sheath of sclerotic cells around the 
central vascular cylinder. On the whole, the roots of mono- 
cotyledons, and especially palms, appear to be better provided 
with such protective layers than those of dicotyledons, a fact 
that may be connected with the looser structure of their 
vascular cylinder. 
The first obstacle to be overcome is the cork. It is im- 
probable that the ferment secreted by the haustorium has any 
great power of dissolving cuticularised membranes, cork 
indeed being, usualy, a successful protection of the invaded 
tissues of the host from further attack, and some other way 
of removing the cork is needed. In paragraph 3 of this paper 
it was suggested that the firm adhesion of the young haustorium 
to a host’s root may be due to the secretion of a gummy 
substance. As the haustorium increases in width, a shearing 
action is thus brought into play, causing the layers of cork to 
be rolled apart (Plate I, figs. 8 and 6). Compare this with 
Sant. II, Plate V, figs. 2 and 2’). In other cases strands of 
cork are lifted bodily away, as shown in Sant. J, figs. 15, 16 
and 18. This action of the superficial layers of the haustorium 
together with the undoubted pressure exerted, is probably 
sufficient to account for the successful penetration of the bark, 
even when composed of thick layers of corky tissue. 
The sclerotic bands appear to be more difficult to nego- 
tiate, especially when they are lignified. We have seen that, 
when they are discontinuous, the sucker, passing along the line 
of least resistance, avoids them and finds its way through the 
parenchymatous tissue between (Plate X, fig. 4, and Sant. JJ, 
