436 PART III. — THE CLASSIFICATION OF PLANTS. 



divisions. The division of the mother-cell is either successive 

 (Monocotyledons, Cycads), or simultaneous (most Dicotyledons 

 and Conifers) ; in the former case the microspores are usually 

 bilateral, in the latter tetrahedral. The form of the mature 

 microspore varies widely; it may be spherical, etc. ; in plants in 

 which pollination takes place under water, the microspore be- 

 comes elongated and filiform {e.g. Zostera, Posidonia, Cymodocea, 

 and to some extent also in Naiaa), and in Halopliila the microspores 

 are aggregated into filaments. 



In some cases the germination of the pollen-grain begins before 

 it is set free from the dehisced pollen- sac, so that it consists of 

 two (sometimes more in Gymnosperms) cells at the time of 

 pollination. 



The macrosporangia, or ovules, are developed singly, or in pairs, 

 or more commonly several together, from a more or less well- 

 developed cushion of tissue, the placenta. When the ovules are 

 borne on sporophylls, the placenta is either marginal, or, less 

 commonly, it is ventral, including the whole of the upper or inner 

 surface of the carpel with, sometimes, the exception of the mid- 

 rib (e.g. Butomus, Nymphaea). When the ovules are borne on 

 the axis, they are either terminal (e.g. Taxus, Polygonum) or 

 lateral (e.g. Primulaceae, Composites). 



The macrosporangium, like the microsporangium, makes its 

 appearance as a small cellular prominence on the surface of the 

 organ which bears it, formed by the division of a group of 

 hypodermal cells ; it is covered at the surface by an epidermal 

 layer which usually remains single, though in some cases (e.g. 

 Gymnosperms, Delphinium, Helianthemum, Rosacea?, Iris Pseuda- 

 corus) the epidermal cells undergo repeated periclinal divisions and 

 thus add considerably to the bulk of the micropylar portion of 

 the nucellus. 



The maci\)sporangium proper (sometimes distinguished as the 

 nucellus) is invested by one or two coats, which grow up from the 

 base, but do not completely close over the apex, leaving there a 

 narrow channel termed the micropyle ; the base of the macro- 

 sporangium, whei^ the coats and the tissue of the sporangium 

 proper become indistinguishable, is termed the chalaza. The 

 coats of the ovule in Phanerogams are the morphological equivalent 

 of the indusium in the Pteridophyta, though, in the former the 

 sorus consists of a single macrospoi'angium ; but this is also the 

 case in Azolla, Isoetes, etc., among the Pteridophyta. 



