ANGIOSPERMS. 



485 



of layers -(the so-called 'special mother-cells'), and these are again enveloped by 

 layers which are common to the whole tetrahedron (Figs. 347 £, 348). If the tetra- 

 hedra have lain for some time in water, the masses of layers usually burst, and the 

 protoplasmic contents of the young pollen-cells are forced out through the fissure, 

 and become rounded off into a sphere (Figs. 344 VII; 347 F, G). Soon after the 

 conversion of the mother-cells of the pollen into a tetrahedron, each protoplasmic 

 mass becomes clothed with a new cell-wall, at first very thin and not continuous 

 with the inner layers of the wall of the mother-cell, as is shown by its becoming 

 detached from them when caused to contract by alcohol. This is the true cell-wall 

 of the pollen, which now increases greatly in thickness, and becomes differentiated 

 into an outer cuticularised layer and an inner one of pure cellulose, the Extine and 

 the Intine. The former becomes covered on the outside with spines (Fig. 348,//^), 

 warts (Fig. 345), ridges, combs, &c. ; w^hile the latter frequently forms considerable 

 thickenings which project inwards at particular spots (Fig. 348, v), and at a later 

 period are employed to form the pollen-tube. During these processes the masses 

 of layers forming the envelope of the tetrahedron become slowly absorbed, their 

 substance is converted into mucilage, and they at length entirely lose their form; 

 their disorganisation may commence either on the inner (as in Fig. 344, VII, x) 

 or outer side (Fig. 348, sg) of the wall of the mother-cell. By the absorption of the 

 chamber in which the young pollen-cells have hitherto been enclosed, they now 

 become free, separate, and fioat in the granular fluid which fills up the cavity of the 

 anther ; and within this they now attain their definite development and size. The 

 fluid being thus used up, the mature pollen-grains finally fill up the cavity of the 

 anther in the form of a powdery mass. 



The ripe pollen-grain of Angiosperms ^ does not undergo any further divisions, 

 like that of Gymnosperms; it remains unicellular; the pollen-tube is developed 

 immediately on the stigma as a protuberance of the intine, which perforates the 

 extine at certain definite spots that have usually been prepared beforehand. The 

 spots where this perforation takes place are often more than one, or even very nume- 

 rous (Fig. 349 a, 350 0) ; yet, notwithstanding the possibility of the formation of this 

 number of pollen-tubes from one grain, only one usually grows to an extent sufficient 

 to effect impregnation. Independently of the sculpture of the extine itself which 

 has already been mentioned, the external form and structure of the outer coat of 

 pollen-grains depends chiefly on the number of the spots at which the perforation 

 takes place, on the mode in which these are arranged, and on the circumstance 

 whether the extine is at these spots merely thinner and the intine projects in the 

 form of a wart (Fig. 349), or whether roundish pieces of the extine become detached 

 in the form of a lid, as in Cucurbitacese and Passiflora (Fig. 37, p. 33), or whether it 

 splits into bands by spiral fissures, as in Thunbergia (Fig. 38, p. 34), &c. At the 

 points of perforation the intine is generally thicker, often forming hemispherical 

 protuberances which furnish the first material for the formation of the pollen-tube 

 (Fig. 350, 0, or the extine only forms thinner longitudinal striae which fold inwards 



» For more minute details see Schacht, Jahrb. ftir wissensch. Bot. II, p. 109, and Luerssen, 

 ibid. VII, p. 34.— [Fritzsche. Beitriige, zur Kenntniss des Pollen, Berlin, 1832.— Mohl, Beitrage zur 

 Anatomic u. Physiologic der Gewiichse, ist Heft, Bern, 1834.] 



