PART IV. CLASSIFICATION. 



The exine is frequently highly differentiated with special refer- 

 ence to the protrusion of the pollen-tubes : it may be porous (e.g. 

 Malvacete, Fig. 236) ; or there may be thin areas at certain points; 

 or (Onagracepe") much-thickened areas where the pollen-tubes are 

 eventually developed ; or, again, areas are marked off here and 

 there which come off like lids under the pressure of the developing 

 pollen-tube (Fig. 235). 



The development of the microspores has already been dealt with 

 in general (see p. 85), so that it will be only necessary here to 

 mention certain special points. The mother-cells of the micro- 

 spores either remain coherent during the development of the 

 microspores, or (as in many Monocotyledons) they become free and 

 float in the granular fluid, derived from the disorganisation of the 

 tapetum, which fills the pollen-sac. The walls of the mother-cells 

 usually become very much thickened, especially in the planes of the 

 future 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 

 becomes elongated and filiform (e.g. Zostera). 



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 Grymnosperms) 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 midrib 

 (e.g. Butomxis, Nymphsea). When the ovules are borne on the 

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

 (e.g. Primulacese, Compositse). 



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 hypo- 

 dermal cells. The macrosporangium proper (sometimes distin- 

 guished 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 



