314 



BOTANY OF THE LIVING PLANT 



off, and the cotyledons expand round the central plumule (Fig. 

 256, III.). Thus the seedling is established. 



On comparing the two Divisions of Seed-Plants substantial simi- 

 larity is seen in the leading facts of form, structure, and physiology. 

 But there are many details which support the geological history of 

 Gymnosperms in showing that they are relatively primitive among 



Seed-bearing Plants. This is seen in 

 their lower degree of vegetative 

 differentiation, and their less ready 

 adaptability to the surroundings, 

 which gives to these plants their 

 stereotyped appearance. Their sim- 

 pler vascular construction points in 

 the same direction, in particular the 

 absence of vessels from the wood of 

 most of them, which is matched by 

 some members of the primitive Angio- 

 spermic Family of the Magnoliaceae 

 (Drimys). But the clearest indications 

 are seen in the propagative details. 



The chromosome -cycle is as in Angio- 

 sperms. The sporophyte is diploid. 

 Reduction takes place at the tetrad- 

 division of the pollen-mother-cell and 

 the megaspore-mother-cell, while duplica- 

 tion occurs at fertilisation. All the 

 divisions which occur between reduction 

 and fertilisation are haploid, and those 

 cells constitute the gametophyte. Those 

 in the microspore and in the pollen-tube 

 represent the male prothattus ; those in 

 the megaspore or embryo-sac constitute 

 the female prothallus, or endosperm. The most notable point for com- 

 parison with the Angiosperms is that in both cases the cell-divisions 

 that precede fertilisation are more numerous than in Angiosperms. In the 

 pollen-grains the first cells that are obliterated are obviously vestigial, 

 and may be held as representing a vegetative region of the male 

 prothallus. But such facts are in contrast to the simpler method of deposit 

 of the grain directly upon the nucellus. The origin and position of the mega- 

 spore are as in Angiosperms. The difference again lies in the complexity of 

 the contents. For a bulky and definitely formed tissue of the endosperm 

 fills it before fertilisation, while archegonia, with ovum, ventral-canal-cell, and 

 neck are present. This represents a female prothallus, with its sexual organs 

 more definitely formed than in Angiosperms, though resembling those seen 

 in the lower Pteridophytes. The process of fertilisation, and its method by 



FIG. 256. 



Pine seed and germination. I. Median 

 section of seed ; y=micrepolar end. II. 

 germination. III. Ditto later ; s=seed- 

 coat ; e= endosperm; c= cotyledons. w = 

 primary root; &c=hypocotyl. (After Sachs.) 



