96 EMBRYOGENESIS IN PLANTS 



carbohydrate pass into the embryo from the prothallus. In these 

 species, it should also be noted (i) that the growth of the apical region is 

 very slow, (ii) that the first leaf primordia — two opposite leaves — are 

 formed late as compared with L. selago and are only of the status of 

 scale leaves, and (iii) that incipient vascular tissue, which is slow to 

 appear behind the apical meristem, is completely absent from the 

 parenchymatous foot. The relative inactivity of the shoot apex suggests 

 that there is a deficiency in some essential nutrient, that it is being 

 inhibited, or that environmental conditions are unfavourable. 



LYCOPODIUM CERNUUM AND L. INUNDATUM 



In these and related species the prothallus is a massive, cylindrical 

 structure, the lower, obconical, mycorrhizal portion being sunk in the 

 soil, while the upper exposed part bears numerous irregular, green, 

 leaf-like lobes. The initial stages in the embryogeny are as in other 

 lycopods. The cells of the lower or hypobasal tier become enlarged, 

 but a distended foot is not formed. The upper or epibasal tier soon 

 breaks through the prothallial tissue, emerges as a free-growing struc- 

 ture, and enlarges into an undifferentiated tuberous body, which 

 Treub described as the 'protocorm,'^ Figs. 23, 24. This organ is roughly 

 spherical in form, is composed entirely of parenchyma, and is attached 

 to the soil by rhizoidal hairs. A symbiotic fungus is present. On the 

 upper surface of the protocorm, a conical outgrowth develops into a 

 cylindrical green leaf, or protophyll; this may or may not have a 

 vascular strand. Subsequently, other leaves of similar type are formed; 

 these are described as being indefinite both in number and in position. 

 Figs. 23, 24. Only at a relatively late stage can a shoot apex be definitely 

 recognised: its position is close to the last-formed of the irregular 

 sequence of protophylls. Thereafter, leaves arise round the apex in 

 normal phyllotactic sequence, and a typical leafy shoot is formed. 



If, now, we examine these data from the physiological point of view, 

 some interesting points emerge. Treub's illustrations show that carbo- 

 hydrates are abundantly present in the massive prothallus and he calls 

 attention to the presence of 'grains d'amidon ... en grande quantite' 

 (Fig. 23d; also Fig. 7d). Again, the histological details indicate that 

 metabolites from the prothallus reach the embryo apex by way of the 

 suspensor and foot. The foot does not show any great amount of cell 

 division but its cells enlarge considerably and starch is deposited in 

 them. It may accordingly be inferred that sugars are passing into the 

 embryo more rapidly than they can be used. When the embryo 

 emerges from the prothallus it forms green protophylls and has therefore 



^ It has been suggested that the term should be discarded as being misleading: it is retained here 

 as a handy descriptive term. 



