Cavers: Xotes on Yorkshire Bryophytes. 



213 



insertion to the thallus, the cells are usually living' and capable 

 of division, so that, except in the oldest parts of the thallus, 

 the scale continues to g^row in breadth. Here and there among- 

 the dark-coloured cells there occur cells with lighter or even 

 colourless walls; each of ihese cells contains an oil-body 

 (Fig. 2, C). 



The RHizoiDS which spring from the lower surface of the 

 midrib are of two kinds, some having smooth walls, whilst in 

 others the inner surface of the cell-wall shows numerous peg- 

 like ingrowths ; the latter are usually distinguished as tubercu- 

 late rhizoids. The smooth-walled rhizoids spring from the sides 

 of the midrib and pass straight down into the soil where their 

 ends often become branched. Most of the tuberculate rhizoids 

 grow from the basal cells of the scales themselves, the rest 

 from the adjacent surface-cells of the midrib, and they run back 

 and join to form a bundle which occupies a median longitudinal 

 groove on the surface of the midrib (Fig. 2, A.)- The tubercu- 

 late rhizoids generally end freely in this median bundle, and 

 their ends do not become branched. Each rhizoid is formed by 

 the continued growth of a single superficial cell, and does not 

 show any divisions. The smooth rhizoids serve to attach the 

 plant to the soil and also to absorb water, whilst the tuberculate 

 ones appear to have the function of storing water, which is 

 contained both inside these rhizoids and in the capillary spaces 

 between them. The scales cover the tuberculate rhizoids and 

 thus protect them against evaporation and further increase their 

 capacity of retaining water by means of capillarity. 



As might be expected, plants of different habitat are found 

 to differ in internal structure as well as in external form. It 

 will be sufficient to compare with the account just given of 

 a typical plant (one growing in open but fairly moist surround- 

 ings) the structural characters found in (1) a plant growing in 

 a drier and more exposed situation, (2) one growing in a moister 

 and more shaded situation, (i) In plants of xerophilous habitat 

 the compact tissue is greatly developed, so that the midrib 

 becomes fleshy in texture, serving to store water and reserve 

 food ; the epidermal cells have strongly thickened walls and 

 contain little or no chlorophyll ; the pores are very narrow ; the 

 rhizoids are abundant, and those of the tuberculate type have 

 the projections on the inner wall-surface long and often branched 

 at the ends ; and there is a deep purple colouration in the 

 ventral scales and in all the tissue lying towards the ventral 

 surface and at the margins of the thallus. (2) On the other 



1904 July 1. 



