PHANEROGAMIA 169 



Pteridopliyta much more nearly than the other forms, especially 

 in the structure of the gametophyte and the differentiation of the 

 sexual organs. 



The Microsporophylls, and their Sporangia. — The mioro- 

 sporophyll varies a good deal in its form ; in the Gymaosperms 

 it may be a flattened leaf with sporangia on its under surface ; 

 it may be a peltate scale, something like that of the flower of 

 Bquisetum, or it may be a short branched axis bearing a variable 

 number of anther lobes. In the Angiosperms it is the structure 

 already described as a stamen, consisting of a filament, carrying 

 at its apex a swollen head or anther. 



The Microsporangia are distributed in various ways upon the 

 sporophyll ; in most oases in groups which are to be considered 

 as sori, corresponding to the sori in the Ferns and their aUies. 

 In the Gymnosperms the flattened staminal leaf of the Conifers 

 has a sorus of two sporangia on its back ; the sporophylls of 

 the Cycads bear a number of groups of sporangia, each group 

 representing a sorus. The anther of the Angiosperms contains 

 a sorus of four sporangia, the pollen-sacs. The indusium or 

 membrane covering the sorus in the Ferns is represented 

 doubtfully in the Cupressines, where the sporangia when young 

 are covered by an outgrowth of the under surface of the sporo- 

 phyU. 



Whether the sporangia arise upon a sporophyll or upon the 

 axis, their mode of development is the same. They are 

 eusporangiate, that is, they originate in a group of several cells. 

 In the young anther, at foiu' places, corresponding to the anther 

 lobes, a row of hypodermal cells is seen to be somewhat 

 different from the rest in their mode of dividing. In a trans- 

 verse section of the anther they appear as four cells, as the row 

 runs longitudinally. The hypodermal cell so seen in transverse 

 section cuts off a cell from itself on its exterior face ; this 

 division is followed by another one parallel to the first, so that 

 the original cell orarchesporium is replaced by a row of three, 

 of which the irmermost has come to lie more deeply m the 

 tissue. The outer two take no part in the development of the 

 spores, which arise from the inner one. The one lying next to 

 this is the first-formed part of a nutritive layer which is made to 

 extend round the sporogenous cell by divisions taking place in 

 the tissue next it. This layer is known as the tapetum and 

 has only a, transitory existence. The outermost of the three 

 cells becomes similarly extended, and the walls of those cells, 

 which are towards the exterior become thickened spirally, con- 



