Lecture XVIII. 137 



rise on each side of itself to new cells which are added on to, and 

 increase, the length of the young leaf. 



The cells immediately behind this apical cell divide longitudin- 

 ally and form a mid-rib of narrow cells running down the middle of 

 the leaf, while the sides of the leaf remain but one cell thick. 

 The cells of the mid-rib are long and narrow with thin cell-walls 

 and resemble those of the axial strand of the stem, but are not 

 connected with it. The apical cell of the leaf continues its growth 

 and division for a time : when it ceases the leaf continues to grow 

 for a short time by the enlargement of the individual cells forming 

 it ; but that too ceases soon. The mature leaf has an oval outline, 

 and is attached to the stem by a broad base. It has a pointed 

 apex. The cells on each side of the mid-rib fit together closely, 

 they are somewhat elongated, in the direction parallel to the mid-rib, 

 and in form are polyhedral ; their walls are at first colourless and 

 subsequently brownish. They contain protoplasm in the form of 

 a film lying against their walls and they enclose a large vacuole. 

 In the protoplasmic film there are numbers of chloroplasts. 



At first the protonema alone fixes the stem to the soil but later, 

 as the stem grows in length, rhizoids are developed from some of 

 the lower cortical cells and supplement the protonema. The 

 principal functions of the rhizoids seem those of attachment and 

 reproduction. It is improbable they are of importance in absorp- 

 tion. The whole surface of the moss plant is absorptive. This 

 explains the absence of a continuous conducting system. The 

 rhizoids are similar to the protonema in structure, being composed 

 of cylindrical cells and being branched. Sometimes they give 

 rise to new plants, just in the same manner as the protonema 

 does. 



While the stem is growing in length it has a pointed apex, the 

 extreme point being occupied by the apical cell. When the 

 sexual organs are about to be formed, the apex becomes flattened 

 and broader. On the male shoot the leaves round the apex are 

 reddish in colour and on the rounded apex itself are formed a 

 number of short hairs the paraphyses, and antheridia. The 

 paraphyses are composed of four or five cells attached end to end. 

 The cells at the base are narrow and the distal cells are large so 

 that each hair is club-shaped. The antheridia are not quite so 

 long as the paraphyses. They are elongated sacs on short narrow 

 stalks attaching them to the apex. The wall of the sacs is formed 

 of a single layer of polyhedral cells which contain chloroplasts. 

 The top is closed by a few large colourless cells the lid-cells. 

 The sac is filled with a vast number of minute cubical cells fitting 



