xi LEPTOSPORANGIAT^E HETEROSPORE& 409 



between its upper wall and the indusium. These are the rest- 

 ing cells of a Nostoc-like alga Anab&na Azollce, which is 

 always found associated with this plant. At the same time 

 that the embryo begins to develop, these cells become active, as- 

 sume the characteristic blue-green colour of the growing plant, 

 and divide into short filaments that at first look like short Oscil- 

 larice. The cells soon become rounded, and heterocysts are 

 formed. Some of these filaments remain entangled about the 

 stem apex of the embryo, while others creep into special cav- 

 ities which are found in all the leaves except the cotyledon, and 

 here develop into a colony. 



The first branch is formed after the plant has developed 

 about eight leaves, but whether its position is constant was not 

 determined 



THE MATURE SPOROPHYTE 



Strasburger (6) has investigated very completely the tissues 

 of the mature sporophyte of Azolla, and Pringsheim ( i ) has 

 done the same in Salvinia, so that these points are very satis- 

 factorily understood. 



The growing point of the stem in Azolla (Fig. 240, A) is 

 curved upward and backward, in Salvinia (Fig. 238, A) it is 

 nearly horizontal. In both genera there is a two-sided apical 

 cell from which segments arise right and left. Each segment 

 divides into a dorsal and ventral cell, and a transverse section 

 just back of the apex shows four cells arranged like quadrants 

 of a circle. In Azolla the dorsal cells develop the leaves, the 

 ventral ones the branches and roots. Each semi-segment is 

 divided into an acroscopic and basiscopic cell, and these are fur- 

 ther divided into a dorsal and lateral cell in the upper ones, into 

 a ventral and lateral one in the lower. The leaves arise from 

 one of the dorsal cells, which may be either acroscopic or basi- 

 scopic, but is always constant on the same side of the shoot, so 

 that the two rows of leaves alternate. The lateral buds, which 

 do not seem to appear at definite intervals, arise from one of the 

 upper cells of the ventral segment, and alternate with the leaves 

 on the same side of the stem. 



The mother cell of a leaf is distinguished by its size and 

 position (Fig. 240, B, III, L), and the first division wall, as in 

 the cotyledon, divides it into two nearly equal lobes. No trace 



