THE SEED-HABIT 333 



plants. As a general rule (e.g. most Conifers and Flowering 

 Plants), however, the vascular system of the ovule stops short 

 at its base. The elaborate vascular supply of the early seed- 

 types, as contrasted with the more modern ones, may perhaps 

 be related to the possession of motile spermatozoids, and to 

 the relative degree of exposure and size of the ovules. 



One important aspect of the permanent retention of the 



Vti 



FIG. IQIB. Reconstruction of a longitudinal section through the seed of 

 Lyginopteris oldhamia (after Oliver). CM., cupule ; /., integument; 

 /.&., bundle traversing integument ; m., megaspore ; n., nucellus ; 

 PC., pollen chamber ; Vb., bundle traversing cupule. 



megaspore, involving considerable economy, is the possibility of 

 postponing the accumulation of food-reserves to a relatively late 

 stage, when fertilisation has taken place and the embryo has 

 begun its development. The embryo is, moreover, more ade- 

 quately protected, during its early phases, than is possible in 

 Pteridophyta and Bryophyta, and, after attaining a certain 

 stage, remains dormant and securely shielded until conditions 

 arise suitable for its further growth. 



